amd64_edac: Check ECC capabilities initially

Rework the code to check the hardware ECC capabilities at PCI probing
time. We do all further initialization only if we actually can/have ECC
enabled.

While at it:
0. Fix function naming.
1. Simplify/clarify debug output.
2. Remove amd64_ prefix from the static functions
3. Reorganize code.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>