sfc: Include XGXS in XMAC link status check except in XGMII loopback

The XGXS block may not get a link immediately in XGXS or XAUI loopback
modes, so we still need to check it.  Split falcon_xaui_link_ok() into
falcon_xgxs_link_ok(), which checks only the Falcon XGXS block, and
falcon_xmac_link_ok(), which checks one or both sides of the link as
appropriate.  Also rename falcon_check_xaui_link() to
falcon_xmac_link_ok_retry().

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