MIPS: checksum: fix sparse flooding on asm/checksum.h

csum_fold() in MIPS' asm/checksum.h is another source of sparse flooding
when building different networking source code.
The thing is that only half of __wsum <--> u32 casts inside the function
is forced, which is insufficient.
Add all necessary forced typecasting to stop floods and simplify actual
bug hunting.
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/checksum.h

@@ -113,9 +113,9 @@ static inline __sum16 csum_fold(__wsum csum)
 	u32 sum = (__force u32)csum;
 
 	sum += (sum << 16);
-	csum = (sum < csum);
+	csum = (__force __wsum)(sum < (__force u32)csum);
 	sum >>= 16;
-	sum += csum;
+	sum += (__force u32)csum;
 
 	return (__force __sum16)~sum;
 }