usb: dwc3: gadget: don't clear EP_BUSY too early

In case of non-Isochronous transfers, we don't
want to clear DWC3_EP_BUSY flag until XferComplete
event. That's because XferInProgress was only enabled
so we can recycle TRBs and usb_requests quicker, but
there are still other pending requests being transferred.

In order to make sure we don't allow for another StartTransfer
command while the HW is still processing other transfers,
we must keep DWC3_EP_BUSY flag set and this what this patch
does.

Fixes: f3af3651 (usb: dwc3: gadget: always enable IOC on
	bulk/interrupt transfers)
Cc: <stable@vger.kernel.org> # v3.15+
Reported-by: sundeep subbaraya <sundeep.lkml@gmail.com>
Tested-by: sundeep subbaraya <sundeep.lkml@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>