Commit 947976d ("[netdevice] Do not force a poll on net_tx()")
requires network devices to have TX rings that are sufficiently large
to allow a transmitted response to all packets received during a
single poll.

Reported-by: Robin Smidsrød <robin@smidsrod.no>
Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>