selftests: drv-net: tso: increase the retransmit threshold

We see quite a few flakes during the TSO test against virtualized
devices in NIPA. There's often 10-30 retransmissions during the
test. Sometimes as many as 100. Set the retransmission threshold
at 1/4th of the wire frame target.

Link: https://patch.msgid.link/20250815224100.363438-1-kuba@kernel.org


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