scsi: megaraid_mbox: remove set but not used variables

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/scsi/megaraid/megaraid_mbox.c: In function 'megaraid_reset_handler':
drivers/scsi/megaraid/megaraid_mbox.c:2580:7: warning:
 variable 'recovering' set but not used [-Wunused-but-set-variable]

drivers/scsi/megaraid/megaraid_mbox.c: In function 'mbox_post_sync_cmd':
drivers/scsi/megaraid/megaraid_mbox.c:2728:12: warning:
 variable 'mbox64' set but not used [-Wunused-but-set-variable]

drivers/scsi/megaraid/megaraid_mbox.c: In function 'megaraid_mbox_support_random_del':
drivers/scsi/megaraid/megaraid_mbox.c:3138:11: warning:
 variable 'mbox' set but not used [-Wunused-but-set-variable]

drivers/scsi/megaraid/megaraid_mbox.c: In function 'megaraid_mbox_flush_cache':
drivers/scsi/megaraid/megaraid_mbox.c:3266:10: warning:
 variable 'mbox' set but not used [-Wunused-but-set-variable]

drivers/scsi/megaraid/megaraid_mbox.c: In function 'megaraid_mbox_fire_sync_cmd':
drivers/scsi/megaraid/megaraid_mbox.c:3302:12: warning:
 variable 'mbox64' set but not used [-Wunused-but-set-variable]

drivers/scsi/megaraid/megaraid_mbox.c: In function 'gather_hbainfo':
drivers/scsi/megaraid/megaraid_mbox.c:3797:10: warning:
 variable 'dmajor' set but not used [-Wunused-but-set-variable]

[mkp: applied by hand due to conflict with hch's DMA cleanup]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>