V4L/DVB (8617): uvcvideo: don't use stack-based buffers for USB transfers.

Data buffers on the stack are not allowed for USB I/O. Use dynamically
allocated buffers instead.

Signed-off-by: Bruce Schmid <duck@freescale.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>