usb: dwc3: gadget: Fail request submission if it was already queued

Function driver should not try to queue a busy request again to DCD
until DCD returns it by calling its completion handler or it is
explicitly dequeued. Some function driver may try to submit a
request again which is with DCD resulting in corruption of request_list
leading to kernel panic later in giveback. Catch such conditions and fail
the request submission to DCD.

Change-Id: Id2a4a1720f61dffc50cc22598734f229caf07ff3
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>