sfc: don't BUG_ON efx->max_channels == 0 in probe

efx_ef10_probe() was BUGging out if the BAR2 size was 0.  This is
 unnecessarily violent; instead we should just fail to probe the device.
Kept a WARN_ON as this problem indicates a broken or misconfigured NIC.

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