fix(mhu): fix compilation error with ENABLE_ASSERTIONS=0 option

After disabling assertion with -DENABLE_ASSERTIONS=0, the build reports
error:

drivers/arm/mhu/mhu_wrapper_v3_x.c: In function 'mhu_get_max_message_size':
drivers/arm/mhu/mhu_wrapper_v3_x.c:448:31: error: variable 'err' set but not used [-Werror=unused-but-set-variable]
         enum mhu_v3_x_error_t err;
                               ^~~

This commit fixes the building failure by making the variable 'err' as
__maybe_unused.

Change-Id: I338e6df03d2f0805c83e96d8e3a4abae41e68678
Signed-off-by: Leo Yan <leo.yan@arm.com>
1 file changed