MIPS: Use CONFIG_CPU_MIPSR2 in csum_partial.S

The csum_partial implementation contain optimalizations for the MIPS R2
instruction set. This optimization is never enabled however because the
if directive uses the CPU_MIPSR2 constant which is not defined anywhere.

Use the CONFIG_CPU_MIPSR2 constant instead.
Signed-off-by: NGabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4971/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/lib/csum_partial.S

@@ -270,7 +270,7 @@ LEAF(csum_partial)
 #endif
 
 	/* odd buffer alignment? */
-#ifdef CPU_MIPSR2
+#ifdef CONFIG_CPU_MIPSR2
 	wsbh	v1, sum
 	movn	sum, v1, t7
 #else
@@ -670,7 +670,7 @@ EXC(	sb	t0, NBYTES-2(dst), .Ls_exc)
 	addu	sum, v1
 #endif
 
-#ifdef CPU_MIPSR2
+#ifdef CONFIG_CPU_MIPSR2
 	wsbh	v1, sum
 	movn	sum, v1, odd
 #else