hv_netvsc: hide warnings about uninitialized/missing rndis device

Hyper-V hosts are known to send RNDIS messages even after we halt the
device in rndis_filter_halt_device(). Remove user visible messages
as they are not really useful.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>