pasemi_mac: Fix TX cleaning

This is a bit awkward. We don't have a timer-delayed interrupt on TX
complete, but we have a count threshold. So set that reasonably high
(32 packets), and schedule the NAPI poll when it goes off. Also bump a
regular timer that will take care of rotting packets for the last 1..31
ones in case we don't trigger a TX interrupt (and there's no RX activity
that would otherwise trigger the poll).

The longer-term fix is to separate TX from RX NAPI and do two separate
poll loops.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>