scsi: core: Introduce scsi_devinfo_key enumeration type

Since symbolic names for the device information keys alread exist,
associate an enumeration type with these symbolic values. This change
makes it clear what the valid values for the 'key' arguments are.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>