mmc: dw_mmc: Make dw_mci_exynos_probe static

Silences the following sparse warning:
drivers/mmc/host/dw_mmc-exynos.c:218:5: warning:
symbol 'dw_mci_exynos_probe' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>