scsi: megaraid_sas: Make poll_aen_lock static

Fix sparse warning:

drivers/scsi/megaraid/megaraid_sas_base.c:187:12:
 warning: symbol 'poll_aen_lock' was not declared. Should it be static?

Link: https://lore.kernel.org/r/20191125144454.22680-1-yuehaibing@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>