scsi: hisi_sas: Fix type casting and missing static qualifier in debugfs code

Sparse can detect some type casting issues in the debugfs code, so fix it
up.

Also a missing static qualifier is added to hisi_sas_debugfs_to_reg_name().

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>