cns3xxx: ethernet - revert: clean up tx descs only when needed

This reverts commit 0772ab938c0aedd7f4cc7127059d6ce8cf929dfa.

Trying to optimize calls to eth_complete_tx in this fasion causes a regression
where when sending only the tx queue can get disabled until a packet is
received. This original call to eth_schedule_poll() is scheduled so it
should not cause a performance issue.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

SVN-Revision: 40592