sfc: Remove efx_nic_type::push_multicast_hash operation

Both implementations of efx_nic_type::reconfigure_mac operation
push the multicast hash filter to the hardware.  It is therefore
redundant to call efx_nic_type::push_multicast_hash as well.

efx_mcdi_mac_reconfigure() also uses this operation, but the
implementation for Siena just uses MCDI anyway.  Merge that into
efx_mcdi_mac_reconfigure().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>