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>

common/spl/spl_mmc.c

@@ -96,7 +96,7 @@ end:
 	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: