The value of 'mask_base' passed to diffwtd_mask is currently
38 for the lowbd path and 42 for the highbd path. Going off of
the mode name (DIFFWTD_38), presumably these are both supposed
to be 38, so change the highbd path accordingly.

Change-Id: I5fb0099c4b8b3ca3c4f211562401b12012f5c002