Remove use case of DC_TX_ONE. It was used to limit queue just 1 TX
frame in DM910x controllers. In r67595(more than 10 years ago) it
was replaced to use "Store and Forward" mode and made controller
generate TX completion interrupt for every frame.

For controllers that have TX interrupt moderation capability,
request TX completion interrupt for every 8-th frames. Previously
dc(4) requested TX completion interrupt if number of queued TX
descriptors is greater than 64. This caused a lot of TX completion
interrupt under high TX load once driver queued more than 64 TX
descriptors. It's quite normal to see more than 64 queued TX
descriptors under high TX load.
This change reduces the number of TX completion interrupts to be
less than 17k under high TX load. Because this change does not
generate TX completion interrupt for each frame, add reclaiming
transmitted buffers in dc_tick not to generate false watchdog
timeouts.
While I'm here add check for queued descriptors in dc_txeof() since
there is no more work to do when there is no pending descriptors.

Make sure to clear status word of TX descriptor in dc_list_tx_init().
Do not update if_opackets if the transmission had failed.

There is no need to execute filter configuration when driver is not
running. Remove wrong driver state change in dc_setfilt_xircom().
While I'm here nuke unnecessary assignments.

Fix a long standing bug where driver handed over RX descriptor
ownership to controller before completion of access to the
descriptor. Driver is faking up status word so it should not give
ownership to controller until it completes RX processing.