ixgbevf: rewrite watchdog task to function similar to igbvf

This patch cleans up the logic dealing with link down/up by breaking down the
link detection and up/down events into separate functions - similar to how these
events are handled in other drivers.

CC: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>