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.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>