pcm/Export: halve the sample rate for DoP

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

src/pcm/PcmExport.cxx

@@ -106,6 +106,11 @@ PcmExport::Params::CalcOutputSampleRate(unsigned sample_rate) const
 		/* DSD_U32 combines four 8-bit "samples" in one 32-bit
 		   "sample" */
 		sample_rate /= 4;
+
+	if (dop)
+		/* DoP packs two 8-bit "samples" in one 24-bit
+		   "sample" */
+		sample_rate /= 2;
 #endif
 
 	return sample_rate;
@@ -117,6 +122,9 @@ PcmExport::Params::CalcInputSampleRate(unsigned sample_rate) const
 #ifdef ENABLE_DSD
 	if (dsd_u32)
 		sample_rate *= 4;
+
+	if (dop)
+		sample_rate *= 2;
 #endif
 
 	return sample_rate;