media: bcm2835-unicam: Fix bug in buffer swapping logic

If multiple sets of interrupts occur simultaneously, it may be unsafe
to swap buffers, as the hardware may already be re-using the current
buffers. In such cases, avoid swapping buffers, and wait for the next
opportunity at the Frame End interrupt to signal completion.

Additionally, check the packet compare status when watching for frame
end for buffers swaps, as this could also signify a frame end event.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>