rco: Clean up casting errors

Fixe a couple of cast errors found by sparse.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/checksum.h

@@ -165,7 +165,8 @@ static inline __wsum remcsum_adjust(void *ptr, __wsum csum,
 	csum = csum_sub(csum, csum_partial(ptr, start, 0));
 
 	/* Set derived checksum in packet */
-	delta = csum_sub(csum_fold(csum), *psum);
+	delta = csum_sub((__force __wsum)csum_fold(csum),
+			 (__force __wsum)*psum);
 	*psum = csum_fold(csum);
 
 	return delta;