x86: mce: Lower maximum number of banks to architecture limit

The Intel x86 architecture right now only supports 32 machine check
banks, more would bump into other MSRs.

So lower the max define to 32.

This only affects a few bitmaps, most data structures are dynamically
sized anyways.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>