scsi: megaraid: make two symbols static in megaraid_mbox.c

scsi: megaraid: make two symbols static in megaraid_mbox.c

Fix the following sparse warning:

drivers/scsi/megaraid/megaraid_mbox.c:305:5: warning: symbol
'dev_attr_megaraid_mbox_app_hndl' was not declared. Should it be static?
drivers/scsi/megaraid/megaraid_mbox.c:315:5: warning: symbol
'dev_attr_megaraid_mbox_ld' was not declared. Should it be static?

Link: https://lore.kernel.org/r/20200407092827.18074-2-yanaijie@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>