usb: dwc3: gadget: move cmd_endtransfer to extra function

This patch adds the extra function __dwc3_stop_active_transfer to
consolidate the same codepath.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20220306211251.2281335-3-m.grzeschik@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>