nfp: expose common functions to be used for AF_XDP

There are some common functionality that can be reused in the upcoming
AF_XDP support. Expose those functions in the header. While at it mark
some arguments of nfp_net_rx_csum() as const.

Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>