sfc: null out channel->rps_flow_id after freeing it

If an ef100_net_open() fails, ef100_net_stop() may be called without
 channel->rps_flow_id having been written; thus it may hold the address
 freed by a previous ef100_net_stop()'s call to efx_remove_filters().
 This then causes a double-free when efx_remove_filters() is called
 again, leading to a panic.
To prevent this, after freeing it, overwrite it with NULL.

Fixes: a9dc3d56 ("sfc_ef100: RX filter table management and related gubbins")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>