net: dsa: qca8k: handle error from qca8k_busy_wait

Propagate errors from qca8k_busy_wait instead of hardcoding return
value.

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