drivers: net: xgene: Make xgene_enet_of_match depend on CONFIG_OF

If CONFIG_NET_XGENE=y but CONFIG_OF=n:

drivers/net/ethernet/apm/xgene/xgene_enet_main.c:1033: warning: ‘xgene_enet_of_match’ defined but not used

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>