Fix rx skb allocation error logging

Signed arithmetic is not required as rtl8169_rx_fill() return belongs
to the [0; NUM_RX_DESC] interval.
Signed-off-by: Jon Mason <jdmason@us.ibm.com>
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>