usb: gadget: uvc: rework complete handler

We refactor the complete handler since the return path with the
locking are really difficult to follow. Just simplify the function by
switching the logic return it on an disabled endpoint early. This way
the second level of indentation can be removed.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20240214-uvc-gadget-cleanup-v1-3-de6d78780459@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>