sfc: Simplify PHY polling

Falcon can generate events for LASI interrupts from the PHY, but in
practice we have never implemented this in reference designs.  Instead
we have polled, inserted the appropriate events, and then handled the
events later.  This is a waste of time and code.

Instead, make PHY poll functions update the link state synchronously
and report whether it changed.  We can still make use of the LASI
registers as a shortcut on the SFT9001.

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