dsa: lan9303: write reg only if necessary

As the regmap_write() is over a slow bus that will sleep, we can speed up
the boot-up time a bit by not bothering to clear a bit that is already
clear.

Signed-off-by: Jerry Ray <jerry.ray@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>