dwc3: gadget: Avoid starting next transfer if transfer is active

XFERNOTREADY event provides status about current transfer. Hence
if transfer is active, then avoid starting next transfer by calling
__dwc3_gadget_kick_transfer() API. Otherwise USB controller stalls to
perform data transfer on that endpoint due to starting new transfer
without completing in progress transfer.

Change-Id: I90370e7e5de55b499a78fbfe5e913f39ec22c7e5
Signed-off-by: Mayank Rana <mrana@codeaurora.org>