Refine the rate selection by choosing the rate
closer to the requested one in case of selecting
single frequency. Previously, the higher rate was
always selected.

Also, fix problem with the best_diff unsigned int
value wrapping (turning negative).
Signed-off-by: NKrzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>