staging: vt6655: upc: Suppress set but not used warning in macro

The variable is used in the function using the macro.

Fixes the following W=1 kernel build warning(s):

 drivers/staging/vt6655/upc.h:45:16: warning: variable ‘byData’ set but not used [-Wunused-but-set-variable]

Cc: Forest Bond <forest@alittletooquiet.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-staging@lists.linux.dev
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210414181129.1628598-28-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vt6655/upc.h

@@ -42,7 +42,7 @@
 
 #define PCAvDelayByIO(uDelayUnit)				\
 do {								\
-	unsigned char byData;					\
+	unsigned char __maybe_unused byData;			\
 	unsigned long ii;					\
 								\
 	if (uDelayUnit <= 50) {					\