net: phy: phylink: Poll link GPIOs

When using a fixed link with a link GPIO, we need to poll that GPIO to
determine link state changes. This is consistent with what fixed_phy.c does.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>