net: dsa: qca8k: move read_switch_id function to common code

The same function to read the switch id is used by drivers based on
qca8k family switch. Move them to common code to make them 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>