sfc: If AN is enabled, always read speed/duplex from the AN advertising bits

When AN is enabled and the link is down the speed/duplex control bits
will not be meaningful.  Use the advertising bits instead, and mask
them with the LPA bits if and only if AN is complete (as before).

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