r8169: improve statistics of missed rx packets

Register RxMissed exists on few early chip versions only, however all
chip versions have the number of missed RX packets in the hardware
counters. Therefore remove using RxMissed and get the number of missed
RX packets from the hardware stats.

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