rtl8139: use net/eth.h macros instead of custom macros

Eliminate the following "custom" macros since they are just duplicates
of net/eth.h macros under a different name:

  ETHER_ADDR_LEN -> ETH_ALEN
  ETH_P_8021Q -> ETH_P_VLAN
  IP_HEADER_LENGTH -> IP_HDR_GET_LEN
  TCP_FLAG_FIN -> TH_FIN
  TCP_FLAG_PUSH -> TH_PUSH

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Message-id: 1438604157-29664-3-git-send-email-stefanha@redhat.com