mmc: add static to spl_mmc_get_device_index()
This function is only used in common/spl/spl_mmc.c[
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>