usb: dwc3: gadget: drop unnecessary loop when cleaning up TRBs

Now that we're using XFERINPROGRESS for all endpoint
types (except Control), we will *always* be completing
one TRB at a time, so it's safe to remove the loop
from dwc3_cleanup_done_reqs.

Signed-off-by: Felipe Balbi <balbi@ti.com>