iwlwifi: keep track of bt coex enable/disable stage

For debugging purpose, keep track of the bt coex enable/disable state.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-agn-debugfs.c b/drivers/net/wireless/iwlwifi/iwl-agn-debugfs.c
index a358d43..a6dbd89 100644
--- a/drivers/net/wireless/iwlwifi/iwl-agn-debugfs.c
+++ b/drivers/net/wireless/iwlwifi/iwl-agn-debugfs.c
@@ -856,6 +856,9 @@
 	if (!iwl_is_alive(priv))
 		return -EAGAIN;
 
+	if (!priv->bt_enable_flag)
+		return -EINVAL;
+
 	/* make request to uCode to retrieve statistics information */
 	mutex_lock(&priv->mutex);
 	ret = iwl_send_statistics_request(priv, CMD_SYNC, false);