Bluetooth: Fix NULL pointer dereference in flush_cb

conn was never set in struct flush_data passed to flush_cb.

Change-Id: I2a16479bf7660a96f705d1194245da02ee6eeff5
Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
diff --git a/net/bluetooth/att.c b/net/bluetooth/att.c
index de9b3f9..d9f951d 100644
--- a/net/bluetooth/att.c
+++ b/net/bluetooth/att.c
@@ -1023,6 +1023,7 @@
 		return BT_ATT_ERR_UNLIKELY;
 	}
 
+	data.conn = conn;
 	data.flags = flags;
 
 	/* Apply to the whole database */