ata: sata_mv, avoid trigerrable BUG_ON

There are several reports that the BUG_ON on unsupported command in
mv_qc_prep can be triggered under some circumstances:
https://bugzilla.suse.com/show_bug.cgi?id=1110252
https://serverfault.com/questions/888897/raid-problems-after-power-outage
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1652185
https://bugs.centos.org/view.php?id=14998

Let sata_mv handle the failure gracefully: warn about that incl. the
failed command number and return an AC_ERR_INVALID error. We can do that
now thanks to the previous patch.

Remove also the long-standing FIXME.

[v2] use %.2x as commands are defined as hexa.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-ide@vger.kernel.org
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/ata/sata_mv.c

@@ -2098,12 +2098,10 @@ static enum ata_completion_errors mv_qc_prep(struct ata_queued_cmd *qc)
 		 * non-NCQ mode are: [RW] STREAM DMA and W DMA FUA EXT, none
 		 * of which are defined/used by Linux.  If we get here, this
 		 * driver needs work.
-		 *
-		 * FIXME: modify libata to give qc_prep a return value and
-		 * return error here.
 		 */
-		BUG_ON(tf->command);
-		break;
+		ata_port_err(ap, "%s: unsupported command: %.2x\n", __func__,
+				tf->command);
+		return AC_ERR_INVALID;
 	}
 	mv_crqb_pack_cmd(cw++, tf->nsect, ATA_REG_NSECT, 0);
 	mv_crqb_pack_cmd(cw++, tf->hob_lbal, ATA_REG_LBAL, 0);