flash/nor/stmsmi: fix compile error with clang 12.0.0

The git preliminarily version of clang 12.0.0_r370171
f067bc3c0ad6 reports an error in the expansion of the macro
SMI_READ_REG():

	error: '(' and '{' tokens introducing statement expression
		appear in different macro expansion contexts
		[-Werror,-Wcompound-token-split-by-macro]

Remove one intermediate macro expansion to make clang happy.

Change-Id: I8ae6d9c18808467ba8044d70cbf0a4f76a18d3e6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5958
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>

src/flash/nor/stmsmi.c

@@ -41,9 +41,8 @@
 #include <jtag/jtag.h>
 #include <helper/time_support.h>
 
-#define SMI_READ_REG(a) (_SMI_READ_REG(a))
-#define _SMI_READ_REG(a)			\
-{									\
+#define SMI_READ_REG(a)				\
+({									\
 	int _ret;						\
 	uint32_t _value;				\
 									\
@@ -51,7 +50,7 @@
 	if (_ret != ERROR_OK)			\
 		return _ret;				\
 	_value;							\
-}
+})
 
 #define SMI_WRITE_REG(a, v)			\
 {									\