ravb: expand rx descriptor data to accommodate hw checksum

EtherAVB may provide a checksum of packet data appended to packet data. In
order to allow this checksum to be received by the host descriptor data
needs to be enlarged by 2 bytes to accommodate the checksum.

In the case of MTU-sized packets without a VLAN tag the
checksum were already accommodated by virtue of the space reserved for the
VLAN tag. However, a packet of MTU-size with a  VLAN tag consumed all
packet data space provided by a descriptor leaving no space for the
trailing checksum.

This was not detected by the driver which incorrectly used the last two
bytes of packet data as the checksum and truncate the packet by two bytes.
This resulted all such packets being dropped.

A work around is to disable RX checksum offload
 # ethtool -K eth0 rx off

This patch resolves this problem by increasing the size available for
packet data in RX descriptors by two bytes.

Tested on R-Car E3 (r8a77990) ES1.0 based Ebisu-4D board

v2
* Use sizeof(__sum16) directly rather than adding a driver-local
  #define for the size of the checksum provided by the hw (2 bytes).

Fixes: 4d86d381

 ("ravb: RX checksum offload")
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>