sfc: Refactor checks for invalid filter ID

Nearly every time we call efx_ef10_filter_remove_unsafe, we first check
for EFX_EF10_FILTER_ID_INVALID, in which case we do nothing.  So move
that check into the function, simplifying all the call sites.

Also, change the return type to void, since none of the callers check it.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>