amd64_edac: Sanitize channel extraction

Cleanup and simplify f10_determine_channel(); make it more readable.
Also drop f10_map_intlv_en_to_shift() in favor of simply counting the
bits in F1x124[DramIntlvEn] which is equivalent.

There should be no functionality change resulting from this patch.

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