mg_disk: fix dependency on libata

Add local copies of ata_id_string() and ata_id_c_string() to mg_disk
so there is no need for the driver to depend on ATA and SCSI.

[ Impact: break dependency on libata by copying ata id string functions ]

Cc: unsik Kim <donari75@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>