ALSA: hdspm - Introduce hdspm_get_aes_sample_rate()

Helper function to return the AES sample rate class. This class needs to
be translated via HDSPM_bit2freq() to get the more common
representation.

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