firewire net: Clear dev->broadcast_rcv_context and dev->broadcast_state after destruction of context.

Clear dev->broadcast_rcv_context to NULL and set dev->broadcast_state
to FWNET_BROADCAST_ERROR after descruction of broadcast context.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NStefan Richter <stefanr@s5r6.in-berlin.de>