igbvf: force link checking when mailbox timeout has occurred

This change forces the link down when a mailbox timeout has occurred.
Previously it was possible for a mailbox timeout to occur but for the
interface to stay up.  The problem with this was that it became possible
for an interface to stay up and miss multiple requests resulting in a
possible issue since the interface will be running in an unknown state.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>