net: phy: fixed-phy: remove fixed_phy_update_state()

mvneta is the only user of fixed_phy_update_state(), which has been
converted to use phylink instead.  Remove fixed_phy_update_state().

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