firewire: core: remove unused variable

firewire: core: remove unused variable

which caused gcc 4.6 to warn about
    variable 'destination' set but not used.

Since the hardware ensures that we receive only response packets with
proper destination node ID (in a given bus generation), we have no use
for destination here in the core as well as in upper layers.

(This is different with request packets.  There we pass destination node
ID to upper layers because they may for example need to check whether
this was an unicast or broadcast request.)

Reported-and-Tested-By: Justin P. Mattock <justinmattock@gmail.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>