MIPS: Unify naming style of vendor CP0.Config6 bits

Other vendor-defined registers use the vendor name as a prefix, not an
infix, so unify the naming style of CP0.Config6 bits.

Suggested-by: Maciej W. Rozycki" <macro@linux-mips.org>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Reviewed-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>