net: ethernet: mtk_wed: check update_wo_rx_stats in mtk_wed_update_rx_stats()

Check if update_wo_rx_stats function pointer is properly set in
mtk_wed_update_rx_stats routine before accessing it.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/b0d233386e059bccb59f18f69afb79a7806e5ded.1694507226.git.lorenzo@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>