staging: axis-fifo: Fix kernel-doc return value warnings

The kernel-doc comments for axis_fifo_read() and axis_fifo_write()
describe their return values as free text, which kernel-doc does not
recognize as a return section:

  $ scripts/kernel-doc -Wall -none drivers/staging/axis-fifo/axis-fifo.c
  Warning: drivers/staging/axis-fifo/axis-fifo.c:121 No description
  found for return value of 'axis_fifo_read'
  Warning: drivers/staging/axis-fifo/axis-fifo.c:214 No description
  found for return value of 'axis_fifo_write'

These warnings only show up in a direct kernel-doc invocation or a
W=2 build (-Wall is added to kernel-doc only when KBUILD_EXTRA_WARN
contains 2), which is why W=1 builds appear clean.

Convert the trailing "Returns ..." sentences into Return: sections so
kernel-doc recognizes the existing return value documentation. No
functional change.

Assisted-by: Claude:claude-sonnet-5
Signed-off-by: Babanpreet Singh <bbnpreetsingh@gmail.com>
Link: https://patch.msgid.link/20260712214115.7-1-bbnpreetsingh@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/axis-fifo/axis-fifo.c

@@ -114,7 +114,7 @@ static void reset_ip_core(struct axis_fifo *fifo)
  * operations must be executed atomically, in order and one after the other
  * without missing any.
  *
- * Returns the number of bytes read from the device or negative error code
+ * Return: The number of bytes read from the device or negative error code
  *	on failure.
  */
 static ssize_t axis_fifo_read(struct file *f, char __user *buf,
@@ -207,7 +207,7 @@ end_unlock:
  * we need to lock before checking if the device has available space to avoid
  * any concurrency issue.
  *
- * Returns the number of bytes written to the device or negative error code
+ * Return: The number of bytes written to the device or negative error code
  *	on failure.
  */
 static ssize_t axis_fifo_write(struct file *f, const char __user *buf,