sfc: Minimal support for 40G link speed

Accept and handle 40G link events.

Accept ethtool link settings of speed == 40000 && duplex, and set the
appropriate MCDI PHY capability.

This does not include reporting of 40G media types, as those have not
yet been assigned numbers in the MCDI protocol.

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