r8169: improve rtl8169_get_mac_version

Currently code snippet (RTL_R32(tp, TxConfig) >> 20) & 0xfcf is used
in few places to extract the chip XID. Change the code to do the XID
extraction only once.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>