[vmm][virtio_net] Return ZX_OK for SetPromiscuousMode

This is needed to add this interface to a bridge.

Test: # Deploy fuchsia with debian_guest to device.
 (host) $ fx set terminal.x64 --with //garnet/packages/experimental:debian_guest --release
 (host) $ fx ota
 # Boot the guest. The vmm will create the 'ethv0' device
 # automatically.
 (fuchsia[0]) $ guest launch debian_guest --legacy-net=false
 # Bridge the new ethv0 to the physical NIC (ethp001f6 for me):
 (fuchsia[1]) $ ifconfig bridge ethv0 ethp001f6
 # Observe the bridge is successfully created.
Change-Id: I1f29726798439660dd03cfbd15a6a7f9db79fa56