net: dsa: microchip: add error checking for ksz_pwrite

Add status validation for port register write inside
lan937x_change_mtu. ksz_pwrite and ksz_pread api's are
updated with return type int (Reference patch mentioned
below). Update lan937x_change_mtu with status validation
for ksz_pwrite16().

Link: https://patchwork.kernel.org/project/netdevbpf/patch/20220826105634.3855578-6-o.rempel@pengutronix.de/



Signed-off-by: Rakesh Sankaranarayanan <rakesh.sankaranarayanan@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>