sfc: ef10: Fix assigning negative value to unsigned variable

fix warning reported by smatch:
251 drivers/net/ethernet/sfc/ef10.c:2259 efx_ef10_tx_tso_desc()
warn: assigning (-208) to unsigned variable 'ip_tot_len'
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Acked-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://lore.kernel.org/r/1649640757-30041-1-git-send-email-baihaowen@meizu.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/sfc/ef10.c

@@ -2256,7 +2256,7 @@ int efx_ef10_tx_tso_desc(struct efx_tx_queue *tx_queue, struct sk_buff *skb,
 	 * guaranteed to satisfy the second as we only attempt TSO if
 	 * inner_network_header <= 208.
 	 */
-	ip_tot_len = -EFX_TSO2_MAX_HDRLEN;
+	ip_tot_len = 0x10000 - EFX_TSO2_MAX_HDRLEN;
 	EFX_WARN_ON_ONCE_PARANOID(mss + EFX_TSO2_MAX_HDRLEN +
 				  (tcp->doff << 2u) > ip_tot_len);