sfc: Use consistent types for filter IDs, indices and search depths

Filter IDs are u32 (but never very large) so an ID/error return
value should have type s32.

Filter indices and search depths are never negative, so should
have type unsigned int.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>