scsi: dc395x: Mark 's_stat2' as __maybe_unused

scsi: dc395x: Mark 's_stat2' as __maybe_unused

It's only utilised when debugging is enabled.

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

 drivers/scsi/dc395x.c: In function ‘start_scsi’:
 drivers/scsi/dc395x.c:1359:6: warning: variable ‘s_stat2’ set but not used [-Wunused-but-set-variable]

Link: https://lore.kernel.org/r/20201102142359.561122-19-lee.jones@linaro.org
Cc: Oliver Neukum <oliver@neukum.org>
Cc: Ali Akcaagac <aliakc@web.de>
Cc: Jamie Lenehan <lenehan@twibble.org>
Cc: "C.L. Huang" <ching@tekram.com.tw>
Cc: Erich Chen <erich@tekram.com.tw>
Cc: Kurt Garloff <garloff@suse.de>
Cc: dc395x@twibble.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>