net: phy: mdio-gpio: Remove support for IRQs in platform data

No current devices use IRQs in platform data, so remove support for
it. The MDIO core will also initialise the new bus such that all
addresses are polled, so remove the unneeded re-initialisation.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>