netdevice.h: fix proto_down_reason kernel-doc warning

Fix kernel-doc warning in <linux/netdevice.h>:

../include/linux/netdevice.h:2158: warning: Function parameter or member 'proto_down_reason' not described in 'net_device'

Fixes: 829eb208 ("rtnetlink: add support for protodown reason")
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Acked-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -1784,6 +1784,7 @@ enum netdev_priv_flags {
  *				the watchdog (see dev_watchdog())
  *	@watchdog_timer:	List of timers
  *
+ *	@proto_down_reason:	reason a netdev interface is held down
  *	@pcpu_refcnt:		Number of references to this device
  *	@todo_list:		Delayed register/unregister
  *	@link_watch_list:	XXX: need comments on this one