sfc: ef10: Fix assigning negative value to unsigned variable

sfc: ef10: Fix assigning negative value to unsigned variable

[ Upstream commit b8ff3395fbdf3b79a99d0ef410fc34c51044121e ]

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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>