mca: fix up couple of issues introduced with amd thresholding in r314636

1. There a was a typo in one place where the processor family is checked (16 vs 0x16). Now the checks are consolidated in a single function.
2. Instead of an array of struct amd_et_state objects the code allocated an array of pointers. That was no problem on amd64 where the sizes are the same, but could be a problem on i386.

Reported by: tuexen and others
Tested by: tuexen (earlier version of the fix)
Pointyhat to: avg
MFC after: 5 days
X-MFC with: r314636