ALSA: hdspm - Allow DDS/Varispeed to be set from userspace

The DDS value is the actual physical sample rate. We set it indirectly
when selecting 44100, 48000 and so on via snd_hdspm_hw_params or
hdspm_set_clock_source.

This commit now allows the DDS value to be altered at runtime, thus
speeding up or slowing down the physical sample rate. This is required
for MADI's varispeed that allows for ±12.5% speed adjustment from the
"selected" rate (32kHz, 44100kHz, 48kHz and so on).

Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>