amd64_edac: Use DBAM_DIMM macro

Instead of open-coding it, use the DBAM_DIMM macro in
amd64_csrow_nr_pages() which we have already.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
diff --git a/drivers/edac/amd64_edac.c b/drivers/edac/amd64_edac.c
index cfb7e57..89cd71e 100644
--- a/drivers/edac/amd64_edac.c
+++ b/drivers/edac/amd64_edac.c
@@ -2094,7 +2094,7 @@
 	 * number of bits to shift the DBAM register to extract the proper CSROW
 	 * field.
 	 */
-	cs_mode =  (dbam >> ((csrow_nr / 2) * 4)) & 0xF;
+	cs_mode = DBAM_DIMM(csrow_nr / 2, dbam);
 
 	nr_pages = pvt->ops->dbam_to_cs(pvt, dct, cs_mode) << (20 - PAGE_SHIFT);