drivers/net/oki-semi: Remove the definition of PCH_GBE_ETH_ALEN

PCH_GBE_ETH_ALEN is equal to ETH_ALEN, so we can replace it with
ETH_ALEN.

If they are not equal, it must be a bug, since this is ethernet,
and the address has been already stored to mc_addr_list as ETH_ALEN
bytes when call pch_gbe_mac_mc_addr_list_update.

Signed-off-by: RongQing.Li <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>