mvpp2: sync only the received frame

In the RX path we always sync against the maximum frame size for that pool.
Do the DMA sync and the unmap separately, so we can only sync by the
size of the received frame.

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>