phylink: merge SGMII and 802.3z handling

The code handling SGMII and 802.3z is essentially the same, except that
we assume 802.3z has no PHY.  Re-organise the code such that these cases
are merged, and exclude 802.3z mode from having a PHY attached.  This
results in the same link handling behaviour as before.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>