netvsc: force link update after MTU change

If two MTU changes are in less than update interval (2 seconds),
then the netvsc network device may get stuck with no carrier.

The netvsc driver debounces link status events which is fine
for unsolicited updates, but blocks getting the update after
down/up from MTU reinitialization.

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