usb: gadget: uvc: move video disable logic to its own function

This patch refactors the video disable logic in uvcg_video_enable
into its own separate function 'uvcg_video_disable'. This function
is now used anywhere uvcg_video_enable(video, 0) was used.

Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Suggested-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Avichal Rakesh <arakesh@google.com>
Link: https://lore.kernel.org/r/20231109004104.3467968-3-arakesh@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>