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

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.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>