sfc: Correct efx_for_each_possible_channel_tx_queue() to skip non-TX channels

efx_for_each_possible_channel_tx_queue() should do nothing for RX-only
or extra channels.  The current definition results in allocating
additional unused hardware TX queues when using the mqprio qdisc and
either separate_tx_channels or SR-IOV.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>