blk: Make blk_next_free_devnum() public

blk_next_free_devnum() can be helpful in some cases. Make it
a public API.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/block/blk-uclass.c

@@ -517,7 +517,7 @@ int blk_find_max_devnum(enum if_type if_type)
 	return max_devnum;
 }
 
-static int blk_next_free_devnum(enum if_type if_type)
+int blk_next_free_devnum(enum if_type if_type)
 {
 	int ret;