media: bcm2835-unicam: Fix for possible dummy buffer overrun

The Unicam hardware has been observed to cause a buffer overrun when
using the dummy buffer as a circular buffer. The conditions that cause
the overrun are not fully known, but it seems to occur when the memory
bus is heavily loaded.

To avoid the overrun, program the hardware with a buffer size of 0 when
using the dummy buffer. This will cause overrun into the allocated dummy
buffer, but avoid out of bounds writes.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>