r8169: use skb_copy_to_linear_data in rtl8169_try_rx_copy

Not a giant leap for mankind, but let's avoid the open-coded memcpy
and use standard helper skb_copy_to_linear_data instead.

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