sfc: support for devlink port requires MAE access

On systems without MAE permission efx->mae is not initialised,
and trying to lookup an mport results in a NULL pointer
dereference.

Fixes: 25414b2a ("sfc: add devlink port support for ef100")
Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/sfc/efx_devlink.c

@@ -626,6 +626,9 @@ static struct devlink_port *ef100_set_devlink_port(struct efx_nic *efx, u32 idx)
 	u32 id;
 	int rc;
 
+	if (!efx->mae)
+		return NULL;
+
 	if (efx_mae_lookup_mport(efx, idx, &id)) {
 		/* This should not happen. */
 		if (idx == MAE_MPORT_DESC_VF_IDX_NULL)