global: Convert simple_strtoul() with decimal to dectoul()

It is a pain to have to specify the value 10 in each call. Add a new
dectoul() function and update the code to use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/net/eth_legacy.c b/net/eth_legacy.c
index 96ed5a4..f383ccc 100644
--- a/net/eth_legacy.c
+++ b/net/eth_legacy.c
@@ -114,7 +114,7 @@
 		return 0;
 
 	/* look for an index after "eth" */
-	index = simple_strtoul(name + 3, NULL, 10);
+	index = dectoul(name + 3, NULL);
 
 	dev = eth_devices;
 	do {