net: phy: mscc: read 'vsc8531, edge-slowdown' as an u32

In the DT binding, it is specified nowhere that 'vsc8531,edge-slowdown'
is an u8, even though it's read as an u8 in the driver.

Let's update the driver to take into consideration that the
'vsc8531,edge-slowdown' 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>