net: dsa: qca8k: handle error with qca8k_rmw operation

qca8k_rmw can fail. Rework any user to handle error values and
correctly return. Change qca8k_rmw to return the error code or 0 instead
of the reg value. The reg returned by qca8k_rmw wasn't used anywhere,
so this doesn't cause any functional change.

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