sfc: Split Falcon-arch-specific and common filter state

Move the common state from struct efx_filter_state into struct efx_nic.
Rename struct efx_filter_state to efx_farch_filter_state and change
the type of efx_nic::filter_state to void *.

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