r8169: improve rtl_rx

This patch improves few aspects of rtl_rx, no functional change intended.

1. inline rtl8169_try_rx_copy
2. make pkt_size unsigned
3. use constant ETH_FCS_LEN instead of value 4
4. We just created the skb, so we don't need the checks in skb_put.
   Also we don't need the return value of skb_put.
   Set skb->tail and skb->len directly.

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