V4L/DVB: 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 when querying control length and control
capabilities.

The control capabilities are now also stored in the uvc_control
structure.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>