MFC r361816: Limit AHCI to only one MSI if more is not needed.

My AMD Ryzen system has 4 AHCI controllers, each supporting 16 MSI vectors.
Since two of the controllers have only one SATA port, limit to single MSI
saves system 30 interrupt vectors for free.

It may be possible to also limit number of MSI vectors to 4 and 8 for the
other two controllers, but according to the AHCI specification after that
controllers may revert to only one vector, that would be a bigger loss to
risk.