sfc: add XDP counters to ethtool stats

Count XDP packet drops, error drops, transmissions and redirects and
expose these counters via the ethtool stats command.

Signed-off-by: Charles McLachlan <cmclachlan@solarflare.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>