[NET] front: Zero negotiated bits in xen_set_features

[NET] front: Zero negotiated bits in xen_set_features

When we reconnect to the backend we need to first zero all negotiated
bits as the functions xen_set_sg and xen_set_tso do not (and are not
supposed to) zero bits when they fail to set them.

This patch also permanently enables the NETIF_F_GSO_ROBUST bit as we
never parse any GSO fields ourselves (even if we did the backend could
not trust us so it's wasted effort).

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>