netdev: Convert MDIO ioctl implementation to use struct mii_ioctl_data

A few drivers still access the arguments to MDIO ioctls as an array of
u16.  Convert them to use struct mii_ioctl_data.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>