OpenSSH: correct logic error in ObscureKeystrokeTiming

Cherry-pick fix:
upstream: when sending ObscureKeystrokeTiming chaff packets, we
can't rely on channel_did_enqueue to tell that there is data to send. This
flag indicates that the channels code enqueued a packet on _this_ ppoll()
iteration, not that data was enqueued in _any_ ppoll() iteration in the
timeslice. ok markus@

OpenBSD-Commit-ID: 009b74fd2769b36b5284a0188ade182f00564136

Obtained from: openssh-portable 146c420d29d0
Reviewed by: gordon
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45823