bss: fix missing config bss_no_flush_when_down

This is Android-specific feature to avoid removing bss on interface
down.

This config is removed unexpectedly in last upstream merge.

Bug: 128935493
Test: manually switch between two access points
Change-Id: I718d375f189d2e3bc974f40382c45c48f1d7707c
diff --git a/wpa_supplicant/config_file.c b/wpa_supplicant/config_file.c
index f1d865c..5fcad36 100644
--- a/wpa_supplicant/config_file.c
+++ b/wpa_supplicant/config_file.c
@@ -1539,6 +1539,10 @@
 	if (config->p2p_interface_random_mac_addr)
 		fprintf(f, "p2p_interface_random_mac_addr=%d\n",
 			config->p2p_interface_random_mac_addr);
+	if (config->bss_no_flush_when_down)
+		fprintf(f, "bss_no_flush_when_down=%d\n",
+			config->bss_no_flush_when_down);
+
 }
 
 #endif /* CONFIG_NO_CONFIG_WRITE */