net: dsa: qca8k: handle error with qca8k_write operation

qca8k_write can fail. Rework any user to handle error values and
correctly return.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>