net: dsa: qca8k: move set age/MTU/port enable/disable functions to common code

The same set age, MTU and port enable/disable function are used by
driver 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>