net: mdio-gpio: make mdiobb_ops const

Make this const as it is only stored in a const field of a
mdiobb_ctrl structure.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>