scsi: fix upper bounds check of sense key in scsi_sense_key_string()

Commit 655ee63c ("scsi constants: command, sense key + additional
sense string") added a "Completed" sense string with key 0xF to
snstext[], but failed to updated the upper bounds check of the sense key
in scsi_sense_key_string().

Fixes: 655ee63c

 ("[SCSI] scsi constants: command, sense key + additional sense strings")
Cc: <stable@vger.kernel.org> # v3.12+
Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>