docs: get rid of :c:type explicit declarations for structs

The :c:type:`foo` only works properly with structs before
Sphinx 3.x.

On Sphinx 3.x, structs should now be declared using the
.. c:struct, and referenced via :c:struct tag.

As we now have the automarkup.py macro, that automatically
convert:
	struct foo

into cross-references, let's get rid of that, solving
several warnings when building docs with Sphinx 3.x.

Reviewed-by: André Almeida <andrealmeid@collabora.com> # blk-mq.rst
Reviewed-by: Takashi Iwai <tiwai@suse.de> # sound
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/driver-api/usb/URB.rst

@@ -47,7 +47,7 @@ called USB Request Block, or URB for short.
 The URB structure
 =================
 
-Some of the fields in struct :c:type:`urb` are::
+Some of the fields in struct urb are::
 
   struct urb
   {

Documentation/driver-api/usb/gadget.rst

@@ -176,9 +176,9 @@ Kernel Mode Gadget API
 
 Gadget drivers declare themselves through a struct
 :c:type:`usb_gadget_driver`, which is responsible for most parts of enumeration
-for a struct :c:type:`usb_gadget`. The response to a set_configuration usually
-involves enabling one or more of the struct :c:type:`usb_ep` objects exposed by
-the gadget, and submitting one or more struct :c:type:`usb_request` buffers to
+for a struct usb_gadget. The response to a set_configuration usually
+involves enabling one or more of the struct usb_ep objects exposed by
+the gadget, and submitting one or more struct usb_request buffers to
 transfer data. Understand those four data types, and their operations,
 and you will understand how this API works.
 
@@ -339,8 +339,8 @@ multi-configuration devices (also more than one function, but not
 necessarily sharing a given configuration). There is however an optional
 framework which makes it easier to reuse and combine functions.
 
-Devices using this framework provide a struct :c:type:`usb_composite_driver`,
-which in turn provides one or more struct :c:type:`usb_configuration`
+Devices using this framework provide a struct usb_composite_driver,
+which in turn provides one or more struct usb_configuration
 instances. Each such configuration includes at least one struct
 :c:type:`usb_function`, which packages a user visible role such as "network
 link" or "mass storage device". Management functions may also exist,

Documentation/driver-api/usb/hotplug.rst

@@ -122,7 +122,7 @@ and their quirks, might have a MODULE_DEVICE_TABLE like this::
 Most USB device drivers should pass these tables to the USB subsystem as
 well as to the module management subsystem.  Not all, though: some driver
 frameworks connect using interfaces layered over USB, and so they won't
-need such a struct :c:type:`usb_driver`.
+need such a struct usb_driver.
 
 Drivers that connect directly to the USB subsystem should be declared
 something like this::