net: marvell: return csum computation result from mvneta_rx_csum/mvpp2_rx_csum

This is a preliminary patch to add hw csum hint support to
mvneta/mvpp2 xdp implementation

Tested-by: Matteo Croce <mcroce@linux.microsoft.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>