Assets: Fix LinearContracts support (#1724)

* Assets: Fix LinearContracts support

* Assets: Fix False positives with IsValid

supportedFlag&a is true for every even number, making the test pretty
close to meaningless.

[This
fix](5b33bc5324)
was a viable fix maintaining bit shifted iota, but there's no benefit to
it at all.

Simplifying.

exchanges/order/order_test.go

@@ -55,7 +55,7 @@ func TestSubmit_Validate(t *testing.T) {
 			Submit: &Submit{
 				Exchange:  "test",
 				Pair:      testPair,
-				AssetType: 255,
+				AssetType: asset.All,
 			},
 		}, // valid pair but invalid asset
 		{