netdevice.h: fix xdp_state kernel-doc warning

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

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

Fixes: 7f0a8382 ("bpf, xdp: Maintain info on attached XDP BPF programs in net_device")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -1849,6 +1849,7 @@ enum netdev_priv_flags {
  *	@udp_tunnel_nic_info:	static structure describing the UDP tunnel
  *				offload capabilities of the device
  *	@udp_tunnel_nic:	UDP tunnel offload state
+ *	@xdp_state:		stores info on attached XDP BPF programs
  *
  *	FIXME: cleanup struct net_device such that network protocol info
  *	moves out.