net: mvneta: fix dma sync size in mvneta_run_xdp

Page pool API will start syncing (if requested) starting from
page->dma_addr + pool->p.offset. Fix dma sync length in
mvneta_run_xdp since we do not need to account xdp headroom

Fixes: 07e13edb ("net: mvneta: get rid of huge dma sync in mvneta_rx_refill")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>