sfc: actually wire up siena_check_caps()

Assign it to siena_a0_nic_type.check_caps function pointer.

Fixes: be904b85

 ("sfc: make capability checking a nic_type function")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>