Update set_working_fdt_addr() to use setenv_addr()

We might as well use this common function instead of repeating the same
code.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_fdt.c b/common/cmd_fdt.c
index 6eec947..ac77a08 100644
--- a/common/cmd_fdt.c
+++ b/common/cmd_fdt.c
@@ -55,12 +55,8 @@
 
 void set_working_fdt_addr(void *addr)
 {
-	char buf[17];
-
 	working_fdt = addr;
-
-	sprintf(buf, "%lx", (unsigned long)addr);
-	setenv("fdtaddr", buf);
+	setenv_addr("fdtaddr", addr);
 }
 
 /*
@@ -347,10 +343,7 @@
 			}
 			if (subcmd[0] == 's') {
 				/* get the num nodes at this level */
-				char buf[11];
-
-				sprintf(buf, "%d", curIndex + 1);
-				setenv(var, buf);
+				setenv_ulong(var, curIndex + 1);
 			} else {
 				/* node index not found */
 				printf("libfdt node not found\n");