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: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>