Revert "usb: dwc3: gadget: remove wait_end_transfer"

This reverts commit 6edcdd0e.

This is a preparation change for merging android-4.19.57 into
msm-4.19 branch.

Reverted due to BootTimeRunner regression.

Change-Id: If6a1340a9ff3f72c412c87a98b6cf94a1b165c7f
Signed-off-by: Ivaylo Georgiev <irgeorgiev@codeaurora.org>