MEDIUM: mux-h1: merge recv_wait and send_wait

This is the same principle as previous commit, but for the H1 mux this
time. The checks in the subscribe()/unsubscribe() calls were factored
and some BUG_ON() were added to detect unexpected cases.
h1_wake_for_recv() and h1_wake_for_send() needed to be refined to
consider the current subscription before deciding to wake up.
1 file changed