i40e: Fix sparse error: uninitialized symbol 'ring'

Init pointer with NULL in default switch case statement.

Previously the error was produced when compiling against sparse.
i40e_debugfs.c:582 i40e_dbg_dump_desc() error: uninitialized symbol 'ring'.

Fixes: 44ea803e ("i40e: introduce new dump desc XDP command")
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/i40e/i40e_debugfs.c

@@ -578,6 +578,9 @@ static void i40e_dbg_dump_desc(int cnt, int vsi_seid, int ring_id, int desc_n,
 	case RING_TYPE_XDP:
 		ring = kmemdup(vsi->xdp_rings[ring_id], sizeof(*ring), GFP_KERNEL);
 		break;
+	default:
+		ring = NULL;
+		break;
 	}
 	if (!ring)
 		return;