mmc: sdhci-omap: make three functions static, fixes warnings

The functions sdhci_omap_set_ios, sdhci_omap_set_power and
sdhci_omap_get_min_clock are local to the source and do not need
to be in global scope, so make them static.

Cleans up sparse warnings:
symbol 'sdhci_omap_set_ios' was not declared. Should it be static?
symbol 'sdhci_omap_set_power' was not declared. Should it be static?
symbol 'sdhci_omap_get_min_clock' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>