sfc: Reinitialise the PHY completely in case of a PHY or NIC reset

In particular, set pause advertising bits properly.

A PHY reset is not necessary to recover from the register self-test,
so use a "invisible" reset there instead.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>