Correctly create new device when device_id is passed to /login (#753)

Fixes https://github.com/matrix-org/dendrite/issues/401

Currently when passing a `device_id` parameter to `/login`, which is [supposed](https://matrix.org/docs/spec/client_server/unstable#post-matrix-client-r0-login) to return a device with that ID set, it instead just generates a random `device_id` and hands that back to you.

The code was already there to do this correctly, it looks like it had just been broken during some change. Hopefully sytest will prevent this from becoming broken again.

clientapi/auth/storage/devices/devices_table.go

@@ -169,6 +169,8 @@ func (s *devicesStatements) selectDeviceByToken(
 	return &dev, err
 }
 
+// selectDeviceByID retrieves a device from the database with the given user
+// localpart and deviceID
 func (s *devicesStatements) selectDeviceByID(
 	ctx context.Context, localpart, deviceID string,
 ) (*authtypes.Device, error) {

clientapi/auth/storage/devices/storage.go

@@ -84,7 +84,7 @@ func (d *Database) CreateDevice(
 	if deviceID != nil {
 		returnErr = common.WithTransaction(d.db, func(txn *sql.Tx) error {
 			var err error
-			// Revoke existing token for this device
+			// Revoke existing tokens for this device
 			if err = d.devices.deleteDevice(ctx, txn, *deviceID, localpart); err != nil {
 				return err
 			}