hv_netvsc: avoid repeated updates of packet filter

The netvsc driver can get repeated calls to netvsc_rx_mode during
network setup; each of these calls ends up scheduling the lower
layers to update tha packet filter. This update requires an
request/response to the host. So avoid doing this if we already
know that the correct packet filter value is set.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>