thunderbolt: Enable plug events

Thunderbolt switches have a plug events capability. This patch adds the
tb_plug_events_active method and uses it to activate plug events during
switch allocation.

Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thunderbolt/tb.h

@@ -19,6 +19,7 @@ struct tb_switch {
 	struct tb_regs_switch_header config;
 	struct tb_port *ports;
 	struct tb *tb;
+	int cap_plug_events; /* offset, zero if not found */
 };
 
 /**