ASoC: i.MX: we can do mono

Whether we can do mono or not depends on the codec. No need
to limit this in the ssi driver.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>