sfc: Synchronise link_advertising and wanted_fc on Siena

All of the ethtool code paths keep them in sync, but we need
to ensure they are sync'd at start of day. Matches the sft9001
driver.

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