ata: sata_mv, avoid trigerrable BUG_ON

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>