net: ethernet: mtk_wed: do not assume offload callbacks are always set

Check if wlan.offload_enable and wlan.offload_disable callbacks are set
in mtk_wed_flow_add/mtk_wed_flow_remove since mt7996 will not rely
on them.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>