Fix Issue #20 - AudioFilterFIR should receive data as read-only blocks

filter_fir.cpp

   // do passthru
   if(coeff_p == FIR_PASSTHRU) {
     // Just passthrough
-    block = receiveWritable(0);
+    block = receiveReadOnly(0);
     if(block) {
       transmit(block,0);
       release(block);
     }
-    block = receiveWritable(1);
+    block = receiveReadOnly(1);
     if(block) {
       transmit(block,1);
       release(block);
     return;
   }
   // Left Channel
-  block = receiveWritable(0);
+  block = receiveReadOnly(0);
   // get a block for the FIR output
   b_new = allocate();
   if(block && b_new) {
   if(b_new)release(b_new);
 
   // Right Channel
-  block = receiveWritable(1);
+  block = receiveReadOnly(1);
   b_new = allocate();
   if(block && b_new) {
     arm_fir_q15(&r_fir_inst, (q15_t *)block->data, (q15_t *)b_new->data, AUDIO_BLOCK_SAMPLES);