times - on every interrupt by using an own set of device methods for the
  IGB class. This translates to introducing igb_if_intr_{disable,enable}()
  and igb_if_{rx,tx}_queue_intr_enable() with that IGB-specific code moved
  out of their EM counterparts and otherwise continuing to use the EM IFDI
  methods also for IGB.
  Note that igb_if_intr_{disable,enable}() also issue E1000_WRITE_FLUSH as
  lost with the conversion of igb(4) to iflib(4).
  Also note, that the em_if_{disable,enable}_intr() methods are renamed to
  em_if_intr_{disable,enable}() for consistency with the names used in the
  interface declaration.
o In em_intr():
  - Don't bother to bail out if the interrupt type is "legacy", i. e. INTx
    or MSI, as iflib(4) doesn't use ift_legacy_intr methods for MSI-X. All
    other iflib(4)-based drivers avoid this check, too.
  - Given that only the MSI-X interrupts have one-shot behavior (by taking
    advantage of the EIAC register), explicitly disable interrupts. Hence,
    em_intr() now matches what {em,igb}_irq_fast() previously did (in case
    of igb(4) supposedly also to work around MSI message reordering errata
    on certain systems).
o In em_if_intr_disable():
  - Clear the EIAC register unconditionally for 82574 and not just in case
    of MSI-X, matching em_if_intr_enable() and bringing back the last hunk
    of r206437 lost with the iflib(4) conversion.
  - Write to EM_EIAC for clearing said register instead of to the IGB-only
    E1000_EIAC used ever since the iflib(4) conversion.

Reviewed by:	shurd
Differential Revision:	https://reviews.freebsd.org/D20176