realtek: l3: reorganize lock access

realtek: l3: reorganize lock access

The layer 3 code shares the register access lock with the DSA driver.
It is currently unclear to which extent this is really needed. To
avoid permanent access to the DSA private structure reorganize the
locking as follows:

- create a lock pointer in the layer 3 control structure
- link it to the DSA lock during probing
- use the ctrl->lock pointer whenever needed.

Link: https://github.com/openwrt/openwrt/pull/23979
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>