i40evf: don't stop watchdog if it hasn't started

If the VF driver fails to complete early init, then rmmod can cause a
softlock when the driver tries to stop a watchdog timer that never even
got initialized.

Add a check to see if the timer is actually initialized before stopping
it.

Change-ID: Id9d550aa8838e07f4b02afe7bc017ef983779efc
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>