staging: bcm2835-codec: Add V4L2_CID_MPEG_VIDEO_B_FRAMES control

FFmpeg insists on trying to set V4L2_CID_MPEG_VIDEO_B_FRAMES to
0, and generates an error should it fail.
As our encoder doesn't support B frames, add a stub handler for
it to silence FFmpeg.

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