realtek: dsa: Fix LAG id allocation

realtek: dsa: Fix LAG id allocation

The rtl83xx_lag_can_offload() function always returned an error because
ds->num_lag_ids was never set. This basically disabled the DSA lag
configuration completely.

Drop the private n_lag variable and instead use the DSA specific one. This
ensures that all the code always has the same reference for the number of
LAGs.

Fixes: 32e5b5ee6b86 ("realtek: Add Link Aggregation (aka trunking) support")
Signed-off-by: Sven Eckelmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20707
Signed-off-by: Hauke Mehrtens <[email protected]>