igb: cleanup interrupt enablement in regards to msix_other

This patch changes a few things so that instead of firing a link status
interrupt directly the get_link_status bit is set and the watchdog is
scheduled.  In addition the mailbox bit is now only enabled if VFs are
enabled.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>