intel: call csum functions with well formatted arguments

The sparse build (C=2) found that there were two drivers
who had not been convered to call the csum_replace_by_diff() function
with sparse clean arguments.  Most if not all drivers force the cast
like this patch does. So these drivers are now joining the party
(a bit late), but with no functional change.
Signed-off-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igbvf/netdev.c

@@ -2056,7 +2056,7 @@ static int igbvf_tso(struct igbvf_ring *tx_ring,
 
 	/* remove payload length from inner checksum */
 	paylen = skb->len - l4_offset;
-	csum_replace_by_diff(&l4.tcp->check, htonl(paylen));
+	csum_replace_by_diff(&l4.tcp->check, (__force __wsum)htonl(paylen));
 
 	/* MSS L4LEN IDX */
 	mss_l4len_idx = (*hdr_len - l4_offset) << E1000_ADVTXD_L4LEN_SHIFT;

drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c

@@ -3814,7 +3814,7 @@ static int ixgbevf_tso(struct ixgbevf_ring *tx_ring,
 
 	/* remove payload length from inner checksum */
 	paylen = skb->len - l4_offset;
-	csum_replace_by_diff(&l4.tcp->check, htonl(paylen));
+	csum_replace_by_diff(&l4.tcp->check, (__force __wsum)htonl(paylen));
 
 	/* update gso size and bytecount with header size */
 	first->gso_segs = skb_shinfo(skb)->gso_segs;