MIPS: memset.S: Disable code unused with non-R6 MIPS configs

This complements commit 8c56208a ("MIPS: lib: memset: Add MIPS R6
support").

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12452/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>