drivers: ethernet: phy: vsc8541: Use 16-bit values for MDIO access

The internal register read/write functions used uint32_t for the values
even though the registers are only 16 bits wide, resulting in a bunch of
casting. Change the internal functions to use uint16_t and wrap them for
the external read/write API which uses uint32_t.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>