core/device: Fix build on some platforms

due to type promotions in C this code was failing on some platforms.

Change-Id: Icf69d7708568f784f0a9d86e8c5ff462ac802fca
Reviewed-on: https://chromium-review.googlesource.com/306772
Reviewed-by: Arman Uguray <armansito@chromium.org>
Tested-by: Arman Uguray <armansito@chromium.org>
diff --git a/src/device.c b/src/device.c
index 6c3a12d..2f73d6a 100644
--- a/src/device.c
+++ b/src/device.c
@@ -2012,7 +2012,7 @@
 
 	sprintf(handle, "%04hx", handle_num);
 	bt_uuid_to_string(&uuid, uuid_str, sizeof(uuid_str));
-	sprintf(value, GATT_CHARAC_UUID_STR ":%04hx:%02hx:%s", value_handle,
+	sprintf(value, GATT_CHARAC_UUID_STR ":%04hx:%02x:%s", value_handle,
 							properties, uuid_str);
 	g_key_file_set_string(key_file, "Attributes", handle, value);