usb: dwc3: gadget: add 'force' argument to stop_active_transfer

It's not always we need to force a transfer to be removed
from the core's internal cache. This extra argument will
help differentiating those two cases.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>