ASoC: imx-ssi: Fix AC97 rates

This device supports multiple rates as described in later AC97
standards. This patch allows playback of different sample frequencies
without conversion.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>