amd64_edac: make driver loading more robust

Currently, the module does not initialize fully when the DIMMs aren't
ECC but remains still loaded. Propagate the error when no instance of
the driver is properly initialized and prevent further loading.

Reorganize and polish error handling in amd64_edac_init() while at it.

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