usb: gadget: u_serial: Dequeue request on gserial_disconnect

Consider a case where, during bus_reset or composition switch,
due to some reason ENDXFER command timed out. Now, gadget driver
won't be giving back the requests to function driver because of
the timeout since controller can still access those requests.
During function_disable gadget driver expects function driver to
dequeue the requests so that the lists maintained by both gadget
driver and function driver should free up all the request.
Currently u_serial driver is just freeing the requests from
read/write pool without any dequeue which is causing
inconsistencies.

Fix this by dequeueing requests from read/write pool on
gserial_disconnect.

Change-Id: I714c7fe681d42a68c864bf0e90748f117b258548
Signed-off-by: Pratham Pratap <quic_ppratap@quicinc.com>