gatt-db: Allow passing Characteristic Value to gatt_db_attribute_get_char_data

This makes gatt_db_attribute_get_char_data work with Characteristic
Value rather than only with Characteristic Declaration.
diff --git a/src/shared/gatt-db.c b/src/shared/gatt-db.c
index 4f5d10b..d3b5cec 100644
--- a/src/shared/gatt-db.c
+++ b/src/shared/gatt-db.c
@@ -1528,7 +1528,7 @@
 	gatt_db_service_foreach(attrib, &characteristic_uuid, func, user_data);
 }
 
-static int gatt_db_attribute_get_index(struct gatt_db_attribute *attrib)
+static int gatt_db_attribute_get_index(const struct gatt_db_attribute *attrib)
 {
 	struct gatt_db_service *service;
 	int index;
@@ -1853,8 +1853,18 @@
 	if (!attrib)
 		return false;
 
-	if (bt_uuid_cmp(&characteristic_uuid, &attrib->uuid))
-		return false;
+	if (bt_uuid_cmp(&characteristic_uuid, &attrib->uuid)) {
+		int index;
+
+		/* Check if Characteristic Value was passed instead */
+		index = gatt_db_attribute_get_index(attrib);
+		if (index < 0)
+			return NULL;
+
+		attrib = attrib->service->attributes[index - 1];
+		if (bt_uuid_cmp(&characteristic_uuid, &attrib->uuid))
+			return false;
+	}
 
 	/*
 	 * Characteristic declaration value: