wifi: mwifiex: Use list_count_nodes()

mwifiex_wmm_list_len() is the same as list_count_nodes(), so use the latter
instead of hand writing it.

Turn 'ba_stream_num' and 'ba_stream_max' in size_t to keep the same type
as what is returned by list_count_nodes().

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/e77ed7f719787cb8836a93b6a6972f4147e40bc6.1682537509.git.christophe.jaillet@wanadoo.fr