mdio: Move allocation of interrupts into core

Have mdio_alloc() create the array of interrupt numbers, and
initialize it to POLLING. This is what most MDIO drivers want, so
allowing code to be removed from the drivers.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>