mmc: sdhci-of-at91: make function sdhci_at91_set_uhs_signaling static

The function sdhci_at91_set_uhs_signaling  is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'sdhci_at91_set_uhs_signaling' 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>