ASoC: pcm179x: Support continuous rates

According to the PCM179x data sheets sampling frequencies between 10
kHz and 200 kHz are supported. Specify support in the driver.

Tested with PCM1791A.

References:
http://www.ti.com/lit/ds/symlink/pcm1791a.pdf
http://www.ti.com/lit/ds/symlink/pcm1792a.pdf
http://www.ti.com/lit/ds/symlink/pcm1795.pdf
http://www.ti.com/lit/ds/symlink/pcm1796.pdf



Signed-off-by: Jacob Siverskog <jacob@teenage.engineering>
Reviewed-by: Johan Hovold <johan@kernel.org>
Acked-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Mark Brown <broonie@kernel.org>