phy: add phy_interface_mode_is_8023z() helper

Add and use phy_interface_mode_is_8023z() helper to identify the
interface modes that use 802.3z negotiation.  Use it in phylink's
phylink_mac_an_restart().

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>