ALSA: hdspm - Allow the TCO and SYNC-IN to be used in slave mode

When using the additional Time Code Option module in slave mode or the
SYNC-In wordclock connector, the sample rate needs to be returned by
hdspm_external_sample_rate().

Since this sample rate may contain any value with 1Hz granularity, we
need to round it to a common rate as done by the OSX driver.

[Fixed missing function declarations by tiwai]

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