net: mvmdio: disable interrupt if resource size is too small

Disable the MDIO interrupt, falling back to polled mode, if the resource
size does not allow us to access the interrupt registers.  All current
DT bindings use a size of 0x84, which allows access, but verifying it is
good practice.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>