firewire net: Omit checking dev->broadcast_rcv_context in fwnet_broadcast_start().

dev->broadcast_rcv_context is always non-NULL if dev->broadcast_state is
not FWNET_BROADCAST_ERROR.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>