mg_disk: Add missing ready status check on mg_write()

When last sector is written, ready bit of status register should be
checked.

Signed-off-by: unsik Kim <donari75@gmail.com>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>