mmc: sdhci-msm: Factor out function to set/get msm clock rate

Factor out msm_set/get_clock_rate_for_bus_mode for it's later
use in changing the tuning sequence for selecting HS400
bus speed mode.

Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
Tested-by: Jeremy McNicoll <jeremymc@redhat.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>