sfc: use efx_channel_tx_[old_]fill_level() in Siena/EF10 TX datapath

Instead of open-coding the calculation with efx_tx_queue_partner(), use
 the functions that iterate over numbers of queues other than 2 with
 efx_for_each_channel_tx_queue().

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