sfc: move initialisation of efx->filter_sem to efx_init_struct()

efx_probe_filters() has not been called yet when EF100 calls into
 efx_mcdi_filter_table_probe(), for which it wants to take the
 filter_sem.

Fixes: a9dc3d56 ("sfc_ef100: RX filter table management and related gubbins")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Link: https://lore.kernel.org/r/24fad43e-887d-051e-25e3-506f23f63abf@solarflare.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>