w1: fix data abort if no one wire bus master present

When the "w1 bus" command is used with no bus master present
a data abort may occur.

This is because uclass_first_device() returns zero, but sets the output
struct udevice pointer to NULL in the no device found case.

Fix w1_get_bus() to account for this and return an error code
as is expected by the callers.

Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
Reviewed-by: Eugen Hristev <eugen.hristev@microchip.com>

drivers/w1/w1-uclass.c

@@ -115,17 +115,19 @@ int w1_get_bus(int busnum, struct udevice **busp)
 	struct udevice *dev;
 
 	for (ret = uclass_first_device(UCLASS_W1, &dev);
-	     !ret;
-	     uclass_next_device(&dev), i++) {
-		if (ret) {
-			debug("Cannot find w1 bus %d\n", busnum);
-			return ret;
-		}
+	     dev && !ret;
+	     ret = uclass_next_device(&dev), i++) {
 		if (i == busnum) {
 			*busp = dev;
 			return 0;
 		}
 	}
+
+	if (!ret) {
+		debug("Cannot find w1 bus %d\n", busnum);
+		ret = -ENODEV;
+	}
+
 	return ret;
 }