net: dsa: qca8k: move mib init function to common code

The same mib function is used by drivers based on qca8k family switch.
Move it to common code to make it accessible also by other drivers.

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