pcm/Export: halve the sample rate for DoP

Move this sample rate fixup from the ALSA output plugin to PcmExport,
where it belongs.

test/test_pcm_export.cxx

@@ -175,6 +175,9 @@ PcmExportTest::TestDop()
 	PcmExport::Params params;
 	params.dop = true;
 
+	CPPUNIT_ASSERT_EQUAL(params.CalcOutputSampleRate(705600u), 176400u);
+	CPPUNIT_ASSERT_EQUAL(params.CalcInputSampleRate(176400u), 705600u);
+
 	PcmExport e;
 	e.Open(SampleFormat::DSD, 2, params);