`namespaceUnderDevice` option of svd2swift defines type in extension of itself

[Rauhul Varma]

namespaceUnderDevice currently produces the following code:

extension ExamplePeripheral {
  /// An example peripheral
  @RegisterBlock
  struct ExamplePeripheral {
    /// An example register
    @RegisterBlock(offset: 0x20)
    var exampleregister: Register<ExampleRegister>
  }
}

extension ExamplePeripheral should instead be extension ExampleDevice