staging: bcm2835-codec: Correct buffer number change on start streaming

"cac8c904 staging: vc04_service: codec: Allow start_streaming to update
the buffernum" allowed the number of buffers configured to be decreased
as well as increased, but there is no requirement for all buffers to
have been queued when start_streaming is called.

Only allow increasing the buffernum.

Fixes: "cac8c904 staging: vc04_service: codec: Allow start_streaming to update the buffernum"
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>