mg_disk: use defines from <linux/ata.h>

While at it:
- remove MG_REG_HEAD_MUST_BE_ON define
- remove MG_REG_CTRL_INTR_ENABLE define
- remove MG_REG_HEAD_LBA_MODE define
- remove unused defines

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>