net: allow user to change NETIF_F_HIGHDMA

NETIF_F_HIGHDMA is like any other TX offloads, so allow user to toggle it.
This is needed later for bridge and bonding convertsion to hw_features.

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