powerpc: Reference parameter in MSR_TM_ACTIVE() macro

Unlike the other MSR_TM_* macros, MSR_TM_ACTIVE does not reference or
use its parameter unless CONFIG_PPC_TRANSACTIONAL_MEM is defined. This
causes an 'unused variable' compile warning unless the variable is also
guarded with CONFIG_PPC_TRANSACTIONAL_MEM.

Reference but do nothing with the argument in the macro to avoid a
potential compile warning.
Signed-off-by: Christopher M. Riedl <cmr@codefail.de>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210227011259.11992-5-cmr@codefail.de

arch/powerpc/include/asm/reg.h

@@ -124,7 +124,7 @@
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
 #define MSR_TM_ACTIVE(x) (((x) & MSR_TS_MASK) != 0) /* Transaction active? */
 #else
-#define MSR_TM_ACTIVE(x) 0
+#define MSR_TM_ACTIVE(x) ((void)(x), 0)
 #endif
 
 #if defined(CONFIG_PPC_BOOK3S_64)