net: dsa: qca8k: improve qca8k read/write/rmw bus access

Put bus in local variable to improve faster access to the mdio bus.

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