via-rhine: fix VLAN priority field (PCP, IEEE 802.1p)

Outgoing packets sent by via-rhine have their VLAN PCP field off by one
(when hardware acceleration is enabled). The TX descriptor expects only VID
and PCP (without a CFI/DEI bit).

Peter Boström noticed and reported the bug.

Signed-off-by: Roger Luethi <rl@hellgate.ch>
Cc: Peter Boström <peter.bostrom@netrounds.com>
Signed-off-by: David S. Miller <davem@davemloft.net>