net: make net_device members garp_port and mrp_port conditional

garp_port is only used in net/802/garp.c which is only compiled with
CONFIG_GARP enabled. Same goes for mrp_port which is only used in
net/802/mrp.c with CONFIG_MRP enabled.

Only include the two members in struct net_device if their respective
CONFIG_* is enabled. This saves a few bytes in struct net_device in case
CONFIG_GARP or CONFIG_MRP are not enabled.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>