sfc: Implement generic features interface

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

diff --cc drivers/net/sfc/efx.c
Simple merge

diff --cc drivers/net/sfc/ethtool.c
Simple merge

diff --cc drivers/net/sfc/net_driver.h
Simple merge

diff --cc drivers/net/sfc/rx.c
Simple merge