scsi: mpt3sas: make function _get_st_from_smid static

The function _get_st_from_smid is local to the source and does not need
to be in global scope, so make it static.

Cleans up sparse warning:
symbol '_get_st_from_smid' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>