As it turns out, MSIs are broken with 2820SA so introduce an AAC_FLAGS_NOMSI
quirk and apply it to these controllers [1]. The same problem was reported
for 2230S, in which case it wasn't actually clear whether the culprit is the
controller or the mainboard, though. In order to be on the safe side, flag
MSIs as being broken with the latter type of controller as well. Given that
these are the only reports of MSI-related breakage with aac(4) so far and
OSes like OpenSolaris unconditionally employ MSIs for all adapters of this
family, however, it doesn't seem warranted to generally disable the use of
MSIs in aac(4).
While it, simplify the MSI allocation logic a bit; there's no need to check
for the presence of the MSI capability on our own as pci_alloc_msi(9) will
just fail when these kind of interrupts are not available.
Reported and tested by: David Boyd [1]

MFC after:	3 days
- Fix a bug in the MSI allocation logic so an MSI is also employed if a
  controller supports only a single message. I haven't seen such an adapter
  out in the wild, though, so this change likely is a NOP.
  While at it, further simplify the MSI allocation logic; there's no need
  to check the number of available messages on our own as pci_alloc_msi(9)
  will just fail if it can't provide us with the single message we want.
- Nuke the unused softc of aacch(4).

MFC after:	1 month