ASoC: mxs-saif: convert to clk_prepare/clk_unprepare

The patch converts mxs-saif driver to clk_prepare/clk_unprepare by
using helper functions clk_prepare_enable/clk_disable_unprepare.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Dong Aisheng <b29396@freescale.com>
Cc: Liam Girdwood <lrg@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>