mmc: sdhci-omap: Make sdhci_omap_reset static

Fix sparse warning:

drivers/mmc/host/sdhci-omap.c:788:6: warning:
 symbol 'sdhci_omap_reset' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Faiz Abbas <faiz_abbas@ti.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>