usb: gadget: uvc: use on returned header len in video_encode_isoc_sg

usb: gadget: uvc: use on returned header len in video_encode_isoc_sg

The function uvc_video_encode_header function returns the number of
bytes used for the header. We change the video_encode_isoc_sg function
to use the returned header_len rather than UVCG_REQUEST_HEADER_LEN and
make the encode function more flexible.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20211022093223.26493-1-m.grzeschik@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>