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>