ixgbevf: introduce delay for checking VFLINKS on 82599

VFLINKS.LINKUP bit tends to flap when a DA or SFP+ cable is disconnected.
It can take up to 500 usecs for the LINKUP bit to be correct.

This patch resolves the issue by introducing a delay for 82599 VFs of at
least 500 usecs to make sure the VFLINKS value is correct.

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>