mmc: sh_mobile_sdhi: do not manage PM clocks manually

On sh-mobile platforms the MMC clock frequency for the TMIO MMC unit is
obtained from the same clock, as the one, that runtime power-manages the
controller. The SDHI glue code has to access that clock directly,
bypassing the runtime PM framework, to get its frequency, but it
shouldn't enable or disable it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>