r8169: replace get_protocol with vlan_get_protocol

This patch is basically the same as 6e74d174

 ("r8152: replace
get_protocol with vlan_get_protocol"). Use vlan_get_protocol
instead of duplicating the functionality.

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