net: dsa: microchip: change the size of reg from u8 to u16

The register size for the ksz8 switches is u8 and for ksz9477 series is
u16.  To have common struct for ksz series switches the size of reg is
increased from u8 to u16.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>