msm: wfd: Wake up the poll-er on streamoff

Wake up userspace threads on streamoff.  Otherwise the only way threads
blocked on poll() can wake up is if the fd is closed.

Change-Id: I0de9b51fe97c423382fd848f3a1ea0056091d5a8
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>