[media] touptek: don't DMA at the stack

As warned by smatch:
	drivers/media/usb/gspca/touptek.c:220 reg_w() error: doing dma on the stack (buff)
	drivers/media/usb/gspca/touptek.c:458 configure() error: doing dma on the stack (buff)

This can fail, as the stack may not be in a memory that would
allod DMA. So, use the usb_buf instead.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>