rpm/mkspec-dtb: resolve packaging conflicts better
    
    The merge commit ad1679b2612f left both %ifs in place. Remove the one
    which d26c540d7eed was removing originally.
    
    This fixes errors like:
    dtb-armv7l.spec : error: line 1442: Unclosed %if
    
    The commit also removed SUBPKG_CASE. Reintroduce it, otherwise we see
    shell garbage in the description