hv_netvsc: Switch the data path at the right time during hibernation

When netvsc_resume() is called, the mlx5 VF NIC has not been resumed yet,
so in the future the host might sliently fail the call netvsc_vf_changed()
-> netvsc_switch_datapath() there, even if the call works now.

Call netvsc_vf_changed() in the NETDEV_CHANGE event handler: at that time
the mlx5 VF NIC has been resumed.

Fixes: 19162fd4 ("hv_netvsc: Fix hibernation for mlx5 VF driver")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>