netdevice.h: remove iflink description

Also move 'group' description to match the order of the net_device structure.

Fixes: 7a66bbc9 ("net: remove iflink field from struct net_device")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>