[SCSI] st: Fixup -ENOMEDIUM

Based on the original patch from Hannes Reinecke <hare@suse.de>

Fix st_open() to return -ENOMEDIUM instead of -EIO if no medium is
found.

Signed-off-by: Kai Makisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>