Fix firewire panic when issuing a reply to an unhandled

asynchronous remote dma request (DMA request that the
hardware cannot automatically handle).

sys/dev/firewire/firewire.c
	In fw_rcv(), add missing early return in the error
	path for DMA requests to unregistered regions.

Submitted by:	gibbs
MFC after:	1 week
Sponsored by:	Spectra Logic
MFSpectraBSD:	1110993 on 2015/01/06

sys/dev/firewire/firewire.c

@@ -2030,6 +2030,7 @@ fw_rcv(struct fw_rcv_buf *rb)
 			rb->xfer->hand = fw_xfer_free;
 			if (fw_asyreq(rb->fc, -1, rb->xfer))
 				fw_xfer_free(rb->xfer);
+			return;
 		}
 		len = 0;
 		for (i = 0; i < rb->nvec; i++)