net: dsa: qca8k: move port VLAN functions to common code

The same port VLAN functions are used by drivers based on qca8k family
switch. Move them to common code to make them accessible also by other
drivers.
Also drop exposing busy_wait and make it static.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>