sfc: only schedule asynchronous filter work if needed

Prevent excessive CPU time spent running a workitem with nothing to do.

We avoid any races by keeping the same check in efx_filter_rfs_expire().

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