sfc: Remove unused field left from mis-merge

Commit eedc765c merged changes from
net-2.6 that added and then removed efx_nic::port_num, which was also
added in net-next-2.6.  The end result should be that it is removed,
since it is now unused.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>