ASoC: mxs-saif: clear clk gate first before register setting

Saif needs clear clk gate first before writing registers or the write
will not success.

The original xx_get_mclk function clear clk gate after mclk setting
that may cause the former mclk setting unwork, then the real output
mclk maybe inaccurate.
Placing the clear before setting mclk to avoid such an issue.

We also have to clear clk gate in startup instead of in prepare function.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>