net: phy: mscc: read 'vsc8531,vddmac' as an u32

In the DT binding, it is specified nowhere that 'vsc8531,vddmac' is an
u16, even though it's read as an u16 in the driver.

Let's update the driver to take into consideration that the
'vsc8531,vddmac' property is of the default type u32.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>