firewire: don't respond to broadcast write requests

Contrary to a comment in the source, request->ack of a broadcast write
request can be ACK_PENDING.  Hence the existing check is insufficient.

Debug dmesg before:
AR spd 0 tl 00, ffc0 -> ffff, ack_pending , QW req, fffff0000234 = ffffffff
AT spd 0 tl 00, ffff -> ffc0, ack_complete, W resp
And the requesting node (linux1394) reports an unsolicited response.

Debug dmesg after:
AR spd 0 tl 00, ffc0 -> ffff, ack_pending , QW req, fffff0000234 = ffffffff

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>