Fix sending call member events on leave (#3799)
https://github.com/matrix-org/matrix-js-sdk/pull/3756 changed
the membership update function to await on the next call, but this
meant it never returned and therefore never cleared
`updateCallMembershipRunning`. We therefore didn't send the updated
call member event when leaving, instead sending it whenever the next
poll interval arrived.
This changes it to only await if we are retrying, not if we're just
scheduling the next poll.
Fixes https://github.com/vector-im/element-call/issues/1763
