selftests: drv-net: so_txtime: relax variance bounds

selftests: drv-net: so_txtime: relax variance bounds

The net-next-hw spinners on netdev.bots.linux.dev observe failing
so-txtime-py tests. A review of stdout shows most failures to be
due to exceeding the 4ms grace period. All I saw were within 8ms.
So increase to that.

Double the bounds from 4 to 8ms. This is still is small enough to
differentiate the delays programmed by the test, 10 and 20ms.

Fixes: 5c6baef3885c ("selftests: drv-net: convert so_txtime to drv-net")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20260610170651.1b644001@kernel.org/
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260621200137.1564776-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>