net: thunderbolt: Fix sparse warnings in tbnet_xmit_csum_and_map()

Fixes the following warning when the driver is built with sparse checks
enabled:

main.c:993:23: warning: incorrect type in initializer (different base types)
main.c:993:23:    expected restricted __wsum [usertype] wsum
main.c:993:23:    got restricted __be32 [usertype]

No functional changes intended.
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/thunderbolt/main.c

@@ -991,8 +991,10 @@ static bool tbnet_xmit_csum_and_map(struct tbnet *net, struct sk_buff *skb,
 {
 	struct thunderbolt_ip_frame_header *hdr = page_address(frames[0]->page);
 	struct device *dma_dev = tb_ring_dma_device(net->tx_ring.ring);
-	__wsum wsum = htonl(skb->len - skb_transport_offset(skb));
 	unsigned int i, len, offset = skb_transport_offset(skb);
+	/* Remove payload length from checksum */
+	u32 paylen = skb->len - skb_transport_offset(skb);
+	__wsum wsum = (__force __wsum)htonl(paylen);
 	__be16 protocol = skb->protocol;
 	void *data = skb->data;
 	void *dest = hdr + 1;