ALSA: dice: limit to current sampling transfer frequency

ALSA PCM core has a functionality for rule of PCM substream parameters.
Typically, when userspace opens PCM character devices, each driver adds
its own rules to PCM substream according to design of hardware. When the
userspace executes hw_params ioctl with favorite parameters, the actual
parameters are calculated according to the rules and the given parameters.
Then, the result is returned to userspace.

Currently, ALSA Dice driver has the rule between channels and rates, while
Dice interface design doesn't allow drivers to retrieve all of the
combinations. Dice drivers are just allowed to get current sampling
transfer frequency and the number of multi bit linear audio data channels
in an data block of an AMDTP packet.

This commit purges the rule, and limit PCM substreams to current sampling
transfer frequency, following to the interface design.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>