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

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>