usb: gadget: uvc: rework uvcg_queue_next_buffer to uvcg_complete_buffer

usb: gadget: uvc: rework uvcg_queue_next_buffer to uvcg_complete_buffer

The function uvcg_queue_next_buffer is used different than its name
suggests. The return value nextbuf is never used by any caller. This
patch reworks the function to its actual purpose, by removing the unused
code and renaming it. The function name uvcg_complete_buffer makes it
more clear that it is actually marking the current video buffer as
complete.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20220402232744.3622565-2-m.grzeschik@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>