block: mtip32xx: mtip32xx: Mark debugging variable 'start' as __maybe_unused

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

 drivers/block/mtip32xx/mtip32xx.c: In function ‘mtip_standby_immediate’:
 drivers/block/mtip32xx/mtip32xx.c:1216:16: warning: variable ‘start’ set but not used [-Wunused-but-set-variable]

Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210312105530.2219008-4-lee.jones@linaro.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/mtip32xx/mtip32xx.c

@@ -1213,7 +1213,7 @@ static int mtip_standby_immediate(struct mtip_port *port)
 {
 	int rv;
 	struct host_to_dev_fis	fis;
-	unsigned long start;
+	unsigned long __maybe_unused start;
 	unsigned int timeout;
 
 	/* Build the FIS. */