net: mvmdio: use GENMASK for masks

Cosmetic patch to use the GENMASK helper for masks.

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>