net: dsa: qca8k: tidy for loop in setup and add cpu port check

Tidy and organize qca8k setup function from multiple for loop.
Change for loop in bridge leave/join to scan all port and skip cpu port.
No functional change intended.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>