libqmi: mock_open: mark device as open

We accidentally forgot to mark the mock device as open when we opened it.
This was tripping an assert failure when we tried to close it and thought
it was already closed.

BUG=None
TEST=New version of qmidev_unittest fails better

Change-Id: Ibb3d5f735847f2c416df264532ec76aeefb3c029
diff --git a/src/mock.c b/src/mock.c
index 404facd..4cb8b3f 100644
--- a/src/mock.c
+++ b/src/mock.c
@@ -31,6 +31,8 @@
   struct mock_priv *priv = dev_priv(dev);
   assert(priv);
 
+  priv->is_open = 1;
+
   return 0;
 }