Add support for the aic7859 which is found on the 2930CU.

Break out the detection logic for the aic7855 and properly report
these chips as 7855s instead of 7850s.

The 2940AU_CN is an aic7860 based card, not aic7860.

Not setting CACHETHEN turned out to be a bad idea. It can cause
spurious corruption under heavy PCI load with multiple masters.