media: uvcvideo: Fix assignment inside if condition

Fixes 'do not use assignment in if condition' errors issued by
scripts/checkpatch.pl on drivers/media/usb/uvc/uvc_driver.c

Signed-off-by: Pedro Guilherme Siqueira Moreira <pedro.guilherme@espectro.eng.br>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>