hv_netvsc: disable NAPI before channel close

This makes sure that no CPU is still process packets when
the channel is closed.

Fixes: 76bb5db5

 ("netvsc: fix use after free on module removal")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>