sfc: add and use efx_tx_send_pending in tx.c

Instead of using efx_tx_queue_partner(), which relies on the assumption
 that tx_queues_per_channel is 2, efx_tx_send_pending() iterates over
 txqs with efx_for_each_channel_tx_queue().
We unconditionally set tx_queue->xmit_pending (renamed from
 xmit_more_available), then condition on xmit_more for the call to
 efx_tx_send_pending(), which will clear xmit_pending.  Thus, after an
 xmit_more TX, the doorbell is un-rung and xmit_pending is true.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>