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