sfc: SFT9001: Fix speed reporting in 1G PHY loopback

Instead of disabling AN in loopback, just prevent restarting AN and
override the speed in sft9001_get_settings().

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