Buffer sizes smaller than one cycle are possible, so don't assert that.
Instead, just provide as much samples as fits to the buffer.
If we are driver when this happens, emit a warning (once). Similarly to
ALSA, as driver we produce only one buffer at cycle start, and no new
buffers in process. If the whole cycle doesn't fit into the buffer,
recording probably will be broken and we want some debug when there will
be a bug report about that.