firewire: net: really fix maximum possible MTU

The maximum unicast datagram size /without/ link fragmentation is
4096 - 4 = 4092 (max IEEE 1394 async payload size at >= S800 bus speed,
minus unfragmented encapssulation header).  Max broadcast datagram size
without fragmentation is 8 bytes less than that (due to GASP header).

The maximum datagram size /with/ link fragmentation is 0xfff = 4095
for unicast and broadcast.  This is because the RFC 2734 fragment
encapsulation header field for datagram size is only 12 bits wide.

Fixes: 5d48f00d

('firewire: net: fix maximum possible MTU')
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: David S. Miller <davem@davemloft.net>