sfc: make tx_queues_per_channel variable at runtime

Siena needs four TX queues (csum * highpri), EF10 needs two (csum),
 and EF100 only needs one (as checksumming is controlled entirely by
 the transmit descriptor).  Rather than having various bits of ad-hoc
 code to decide which queues to set up etc., put the knowledge of how
 many TXQs a channel has in one place.

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