- In em_msix_link(), properly handle IGB-class devices after the iflib(4)
  conversion again by only setting EM_MSIX_LINK for the EM-class 82574
  and by re-arming link interrupts unconditionally, i. e. not only in
  case of spurious interrupts. This fixes the interface link state change
  detection for the IGB-class. [1]
- In em_if_update_admin_status(), only re-arm the link state change
  interrupt for 82574 and also only if such a device uses MSI-X, i. e.
  takes advantage of autoclearing. In case of INTx and MSI as well as
  for LEM- and IGB-class devices, re-arming isn't appropriate here and
  setting EM_MSIX_LINK isn't either.
  While at it, consistently take advantage of the hw variable.

PR:	236724 [1]
Differential Revision:	https://reviews.freebsd.org/D21924