dwc3: gadget: Add support for transfer complete events counting

Add counting of USB transfers completion events/interrupts
so that it can be used to determine USB transfers or BUS usage.
This is a simple approach which gives reasonable accuracy to
avoid counting actual bytes that got transferred over USB.

Change-Id: Ib06e916ef1b3a7dbf0b3c720d306e922043a7f33
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>