usb: f_qdss: Flush connect_work in qdss close

When qdss open and close are called back to back at a high rate,
connect_work that gets queued in qdss open can execute after
qdss close. For qdss HW path the connect work that gets scheduled
as part of the next qdss open results in ep config failure as the
ep is already configured during previous connect work. Now before
queuing the request if there is a qdss close it would reset the dbm
ep configuration thus routing this request via software path instead
of dbm path. The subsequent dequeue would lead to a NULL pointer
dereference of completion callback. Fix this by flushing connect_work
as part of qdss close for graceful termination.

Change-Id: I8edf73ea1f87e297297828e6e25c7c14a422ec3a
Signed-off-by: Elson Roy Serrao <eserrao@codeaurora.org>