net: dsa: qca8k: move port set status/eee/ethtool stats function to common code

The same logic to disable/enable port, set eee and get ethtool stats is
used by drivers based on qca8k family switch.
Move it to common code to make it 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>