[ALSA] intel8x0: an attempt to make ac97_clock measurement more reliable

- use monotonic posix clock to measure time
- try to avoid reading zero from PICB (position in current buffer) register
- show also measured samples
- when clock is near 41000 or 44100, use exactly these values
  (they appears to be reference clocks for hardware manufacturers)

Signed-off-by: Jaroslav Kysela <perex@perex.cz>