amd64_edac: Rewrite unganged mode code of f10_early_channel_count

Simplify the procedure by checking if there is any DIMM in each channel.
This patch will fix the bugs such as when there is no DIMMs under
certain node, two DIMMs in the same channel, and only one DIMM in each
channel of the node.

Borislav: minor fixups

Signed-off-by: Wan Wei <wanwei@mail.dawning.com.cn>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>