libertas: Don't set NETIF_F_IPV6_CSUM in dev->features

I'm not sure why it was doing this, and I'm not sure I _want_ to know
why. But calling it NETIF_F_DYNALLOC doesn't change the fact that the
kernel believes it to be NETIF_F_IPV6_CSUM, and that IPv6 communication
is hence buggered.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>