radioInterface: Fix multi-channel buffer index bug

Discrete RF multi-channel was using hard coded buffer index
on the channel iteration for transmit sample conversion. End
result was segmentation fault on dual RF channel devices with
both channels active (Ettus B210 and UmTRX).

Signed-off-by: Tom Tsou <tom.tsou@ettus.com>

Transceiver52M/radioInterface.cpp

@@ -344,7 +344,7 @@ bool RadioInterface::pushBuffer()
 
   for (size_t i = 0; i < mChans; i++) {
     convert_float_short(convertSendBuffer[i],
-                        (float *) sendBuffer[0]->getReadSegment(),
+                        (float *) sendBuffer[i]->getReadSegment(),
                         powerScaling[i],
                         segmentLen * 2);
   }