USB: gadget: qdss: Flush pending works before function unbind

Workqueue might have pending connect/disconnect works when
composite driver calls qdss unbind as part of composition
switch or remove. As qdss_unbind frees up device references
e.g. ep->driver_data, flush pending works from unbind to avoid
any access to them later from work_fn.

Change-Id: I2ea3988ccebb6fc34772b30965ab8a1cb74d5464
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>