ASoC: mxs-saif: fix clk_prepare() without matching clk_unprepare()

The clk_prepare() call in hw_params() has no matching clk_unprepare(),
leaving the clk with an ever-increasing prepare count.  Moreover,
hw_params() can be called multiple times which would again leave us
with a runaway prepare count.  Fix this by moving the clk_prepare()
call to the startup() function and adding a shutdown() function with
a matching clk_unprepare() as these operations are already correctly
bracketed by soc-core.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Mark Brown <broonie@kernel.org>