mdio: remove an unneed condition

It used to be that mdio->irq was a pointer but after e7f4dc35


('mdio: Move allocation of interrupts into core') it's an array inside
the mdio struct so it can never be NULL.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>