sfc: don't try to create more channels than we can have VIs

Calculate efx->max_vis at probe time, and check against it in
 efx_allocate_msix_channels() when considering whether to create XDP TX
 channels.

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