netvsc: fix net poll mode

The ndo_poll_controller function needs to schedule NAPI to pick
up arriving packets and send completions. Otherwise no data
will ever be received. For simple case of netconsole, it also
will allow send completions to happen.  Without this netpoll
will eventually get stuck.

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