net: mvmdio: introduce an ops structure

Introduce an ops structure to add an indirection on the is_done
function, as this is needed to add the xMDIO support later.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>