[libata] catch, and ack, spurious DMA interrupts

[libata] catch, and ack, spurious DMA interrupts

Hardware issue on Intel ICH5 requires an additional ack sequence
over and above the normal IDE DMA interrupt ack requirements.  Issue
described in post to freebsd list:
http://www.mail-archive.com/freebsd-stable@freebsd.org/msg58421.html

Since the bug workaround only requires a single additional PIO or
MMIO read in the interrupt handler, it is applied to all chipsets
using the standard libata interrupt handler.

Credit for research the issue, creating the patch, and testing the
patch all go to Jon Burgess.