sfc: Remove efx_nic_dummy_op_int() as redundant with efx_port_dummy_op_int()

Update comment on the dummy operation implementations.

Line up the board operation initialisers.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>