Remove rate constraints from generic ASoC AC'97 CODEC.

Supported rates should be detected and constrained anyway by
AC'97 generic code - was tested with VT1613 CODEC and iMX6 SSI
controller.

This way this driver can be used for platforms which don't need
specialized AC'97 CODEC drivers while at the same avoiding
code duplication from implementing equivalent functionality in
a controller driver.
Signed-off-by: Maciej Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Mark Brown <broonie@kernel.org>