ASoC: pcm512x: Fix divide by zero issue

If den=1 and pllin_rate>20MHz then den and num are adjusted to 0
causing a divide by zero error a few lines further on. Therefore
this patch correctly scales num and den such that
pllin_rate/den < 20MHz as required in the device data sheet.

Signed-off-by: Howard Mitchell <hm@hmbedded.co.uk>
Signed-off-by: Mark Brown <broonie@sirena.org.uk>
Cc: stable@vger.kernel.org