net: dsa: qca8k: move bridge functions to common code

The same bridge functions are used by drivers based on qca8k family
switch. Move them to common code to make them accessible also by other
drivers.
While at it also drop unnecessary qca8k_priv cast for void pointers.

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