xen: netfront: assume all hw features are available until backend connection setup

We need to assume that all features will be available when registering the
netdev otherwise they are ommitted from the initial set of
dev->wanted_features. When we connect to the backed we reduce the set as
necessary due to the call to netdev_update_features() in xennet_connect().

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>