mmc: sdhci-omap: Update 'power_mode' outside sdhci_omap_init_74_clocks

Updating 'power_mode' in sdhci_omap_init_74_clocks results in
'power_mode' never updated to MMC_POWER_OFF during card
removal. This results in initialization sequence not sent to the
card during re-insertion.
Fix it here by adding sdhci_omap_set_power_mode to update power_mode.
This function can also be used later to perform operations that
are specific to a power mode (e.g, disable tuning during power off).

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>