BCM2835-V4L2: Ensure H264 header bytes get a sensible timestamp

H264 header come off VC with 0 timestamps, which means they get a
strange timestamp when processed with VC/kernel start times,
particularly if used with the inline header option.
Remember the last frame timestamp and use that if set, or otherwise
use the kernel start time.

https://github.com/raspberrypi/linux/issues/1836



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>