r8169: improve handling VLAN tag

The VLAN tag is stored in the descriptor in network byte order.
Using swab16 works on little endian host systems only. Better play safe
and use ntohs or htons respectively.

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