r8169: remove unneeded check in rtl8169_rx_fill

rtl8169_rx_fill() is called only once and directly before the call
array tp->Rx_databuff[] is filled with zero's. Therefore we don't
need this check.

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