ALSA: hdspm - Work around broken DDS value on PCI RME MADI

On PCI RME MADI cards, the PLL register does not contain the proper
value, so the calculated system_sample_rate is wrong. In this case, we
simply return the cached rate from struct hdspm.

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