mmc: sdhci-bcm2835: Staticize bcm2835_sdhci_get_min_clock

'bcm2835_sdhci_get_min_clock' is used only in this file.
Make it static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>