usb: dwc3: gadget: wait for End Transfer to complete

Instead of just delaying for 100us, we should
actually wait for End Transfer Command Complete
interrupt before moving on. Note that this should
only be done if we're dealing with one of the core
revisions that actually require the interrupt before
moving on.

[ felipe.balbi@linux.intel.com: minor improvements ]

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>