i40evf: don't go further down

If the device is down, there's no place to go but up, so don't try to go
down even more. This prevents a CPU soft lock in napi_disable().

Change-ID: I8b058b9ee974dfa01c212fae2597f4f54b333314
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>