e1000: Rework em_msi_link interrupt filter
- Fix 82574 Link Status Changes, carrying the OTHER mask bit around as needed.
- Move igb-class LSC re-arming out of FAST back into the handler.
- Clarify spurious/other interrupt re-arms in FAST.
In MSI-X mode, 82574 and igb-class devices use an interrupt filter to
handle Link Status Changes. We want to do LSC re-arms in the handler
to take advantage of autoclear (EIAC) single shot behavior.
82574 uses 'Other' in ICR and IMS for LSC interrupt types when in MSI-X
mode, so we need to set and re-arm the 'Other' bit during attach and
after ICR reads in the FAST handler if not an LSC or after handling on
LSC due to autoclearing.
This work was primarily done to address the referenced PR, but inspired
some clarification and improvement for igb-class devices once the
intentions of previous bug fix attempts became clearer.