In case of scb allocation failure, mega_build_cmd() can return NULL scb
but set busy, which causes a non-zero return from ->queuecommand() while
the scsi command has been completed.   Just returning busy without calling
done() should be enough for appropriate retries.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>