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>
diff --git a/common/spl/spl_mmc.c b/common/spl/spl_mmc.c
index 18c1b59..bb48cac 100644
--- a/common/spl/spl_mmc.c
+++ b/common/spl/spl_mmc.c
@@ -96,7 +96,7 @@
 	return 0;
 }
 
-int spl_mmc_get_device_index(u32 boot_device)
+static int spl_mmc_get_device_index(u32 boot_device)
 {
 	switch (boot_device) {
 	case BOOT_DEVICE_MMC1: