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>