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>

arch/mips/lib/memset.S

@@ -228,10 +228,12 @@
 	.hidden __memset
 	.endif
 
+#ifdef CONFIG_CPU_MIPSR6
 .Lbyte_fixup\@:
 	PTR_SUBU	a2, $0, t0
 	jr		ra
 	 PTR_ADDIU	a2, 1
+#endif /* CONFIG_CPU_MIPSR6 */
 
 .Lfirst_fixup\@:
 	jr	ra