V4L/DVB (9646): em28xx: avoid allocating/dealocating memory on every control urb

Before this patch, every register setup on em28xx were dynamically
allocating a temporary buffer for control URB's to be handled.

To avoid this ping-pong, use, instead a pre-allocated buffer.

Also, be sure that read control URB's also use the buffer, instead of
relying on a stack buffer.

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