amd64_edac: check only ECC bit in amd64_determine_edac_cap

Checking whether the machine is using ECC enabled DRAM is done through
testing the DimmEccEn bit in the DRAM Cfg Low register (F2x[1,0]90). Do
that instead of testing all bits from the DimmEccEn upwards.

Also, remove mci->edac_cap assignment and use value returned from
amd64_determine_edac_cap().

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