Save one register read (AHCI_IS) for AHCI controllers with only one port.

For controllers with only one port (like PCIe or M.2 SSDs) interrupt can
come from only one source, and skipping read saves few percents of CPU time.

MFC after:	1 month
H/W donated by:	I/O Switch
Add a define for index of IA32_XSS MSR, which is, per SDM rev. 50, an
analog of XCR0 for ring 0 FPU state, used by XSAVES and XRSTORS.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week