ALSA: compress: add num_sample_rates in snd_codec_desc

this gives ability to convey the valid values of supported rates in
sample_rates array

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
(cherry picked from commit 929559be6d2c494e25bb58b730da4a78c1459e7b)
Signed-off-by: Yuchen Song <yuchens@nvidia.com>
Change-Id: Icfbb6d272a70c0a94719613c00bac18c5a0e3f87