scsi_transport_fc: Introduce scsi_host_{get,put}()

Use scsi_host_{get,put}() instead of open-coding these functions.
Compile-tested only.

[mkp: Dropped CC:stable and fixed James Smart's email address]

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