usb: gadget: uvc: consistently use define for headerlen

The uvc request headerlen of 2 was defined as UVCG_REQUEST_HEADER_LEN
in commit e81e7f9a ("usb: gadget: uvc: add scatter gather support").
We missed to use it consistently. This patch fixes that.

Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20211018072059.11465-1-m.grzeschik@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>