libata: array underflow in ata_find_dev()

commit 59a5e266 upstream.

My static checker complains that "devno" can be negative, meaning that
we read before the start of the loop.  I've looked at the code, and I
think the warning is right.  This come from /proc so it's root only or
it would be quite a quite a serious bug.  The call tree looks like this:

proc_scsi_write() <- gets id and channel from simple_strtoul()
-> scsi_add_single_device() <- calls shost->transportt->user_scan()
   -> ata_scsi_user_scan()
      -> ata_find_dev()

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>