sfc: allocate channels for XDP tx queues

Each CPU needs access to its own queue to allow uncontested
transmission of XDP_TX packets. This means we need to allocate (up
front) enough channels ("xdp transmit channels") to provide at least
one extra tx queue per CPU. These tx queues should not do TSO.

Signed-off-by: Charles McLachlan <cmclachlan@solarflare.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>