net: dsa: qca8k: move port FDB/MDB function to common code

The same port FDB/MDB function are used by drivers based on qca8k family
switch. Move them to common code to make them accessible also by other
drivers.
Also drop bulk read/write functions and make them static

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