netvsc: fix hang on netvsc module removal

The code in netvsc_device_remove was incorrectly calling napi_disable
repeatedly on the same element. This would cause attempts
to remove netvsc module to hang.

Fixes: 2506b1dc4bbe ("netvsc: implement NAPI")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>