mmc: sh_mmcif: Convert to clk_prepare|unprepare

Previously only clk_enable|disable were being used. Adapt properly
to the clock API, by also using clk_prepare|unprepare.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>