net, xdp: Add missing xdp_features description

Add missing xdp_features field description in the struct net_device
documentation. This patch fix the following warning:

  [...]
  ./include/linux/netdevice.h:2375: warning: Function parameter or member 'xdp_features' not described in 'net_device'
  [...]

Fixes: d3d854fd ("netdev-genl: create a simple family for netdev stuff")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/7878544903d855b49e838c9d59f715bde0b5e63b.1675705948.git.lorenzo@kernel.org

include/linux/netdevice.h

@@ -1819,6 +1819,7 @@ enum netdev_ml_priv_type {
  *			of Layer 2 headers.
  *
  *	@flags:		Interface flags (a la BSD)
+ *	@xdp_features:	XDP capability supported by the device
  *	@priv_flags:	Like 'flags' but invisible to userspace,
  *			see if.h for the definitions
  *	@gflags:	Global flags ( kept as legacy )