pcm/Export: DSD_U32 quarters the sample rate

DSD_U32 packs four bytes instead of one large "sample", thus the
sample rate is one quarter of the input sample rate.  This fixes a
rather critical DSD_U32 playback problem.

test/test_pcm_export.cxx

@@ -146,6 +146,9 @@ PcmExportTest::TestDsdU32()
 	PcmExport::Params params;
 	params.dsd_u32 = true;
 
+	CPPUNIT_ASSERT_EQUAL(params.CalcOutputSampleRate(705600u), 176400u);
+	CPPUNIT_ASSERT_EQUAL(params.CalcInputSampleRate(176400u), 705600u);
+
 	PcmExport e;
 	e.Open(SampleFormat::DSD, 2, params);