octeon_ep: cancel tx_timeout_task later in remove sequence

tx_timeout_task is canceled too early when removing the driver. Nothing
prevents .ndo_tx_timeout from triggering and queuing the work again.

Better cancel it after the netdev is unregistered.
It's harmless for octep_tx_timeout_task to run in the window between the
unregistration and cancelation, because it checks netif_running.

Fixes: 862cd659

 ("octeon_ep: Add driver framework and device initialization")
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Link: https://lore.kernel.org/r/20230810150114.107765-3-mschmidt@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>