scsi: hisi_sas: Make three functions static

Fix sparse warnings:

drivers/scsi/hisi_sas/hisi_sas_main.c:3686:6:
 warning: symbol 'hisi_sas_debugfs_release' was not declared. Should it be static?
drivers/scsi/hisi_sas/hisi_sas_main.c:3708:5:
 warning: symbol 'hisi_sas_debugfs_alloc' was not declared. Should it be static?
drivers/scsi/hisi_sas/hisi_sas_main.c:3799:6:
 warning: symbol 'hisi_sas_debugfs_bist_init' was not declared. Should it be static?

Link: https://lore.kernel.org/r/20190923054035.19036-1-yuehaibing@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>