usb: dwc3: gadget: Refactor TRB completion handler

To prepare for handling of XferComplete event, let's refactor and split
up dwc3_gadget_endpoint_transfer_in_progress() to handle TRBs completion
for different events. The handling of TRBs completion will be the same,
but the status of XferComplete event is different than XferInProgress.
No functional change in this commit.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>