sfc: assign TXQs without gaps

Since we only allocate VIs for the number of TXQs we actually need, we
 cannot naively use "channel * TXQ_TYPES + txq" for the TXQ number, as
 this has gaps (when efx->tx_queues_per_channel < EFX_TXQ_TYPES) and
 thus overruns the driver's VI allocations, causing the firmware to
 reject the MC_CMD_INIT_TXQ based on INSTANCE.
Thus, we distinguish INSTANCE (stored in tx_queue->queue) from LABEL
 (tx_queue->label); the former is allocated starting from 0 in
 efx_set_channels(), while the latter is simply the txq type (index in
 channel->tx_queue array).
To simplify things, rather than changing tx_queues_per_channel after
 setting up TXQs, make Siena always probe its HIGHPRI queues at start
 of day, rather than deferring it until tc mqprio enables them.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>