s390/checksum: have consistent calculations

Use "|" instead of "+" within csum_fold() for consistency reasons,
like in the rest of the file.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/checksum.h

@@ -54,7 +54,7 @@ static inline __sum16 csum_fold(__wsum sum)
 {
 	u32 csum = (__force u32) sum;
 
-	csum += (csum >> 16) + (csum << 16);
+	csum += (csum >> 16) | (csum << 16);
 	csum >>= 16;
 	return (__force __sum16) ~csum;
 }