ALSA: hdspm - AES32: Enable TCO input in hdspm_external_sample_rate()

This patch adds support to read the TCO sample rate in
hdspm_external_sample_rate() on RME AES(32) cards.

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