net: mvmdio: put the poll intervals in the ops structure

Put the two poll intervals (min and max) in the driver's ops
structure. 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>