net: introduce netdev_change_features()

It will be needed by bonding and other drivers changing vlan_features
after ndo_init callback.

As a bonus, this includes kernel-doc for netdev_update_features().

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>