transceiver: make the transmit drive loop bus dependent

With the introduction of the B100, there is USB support
using UHD devices. The characteristics of the trasmit
side burst submissions are more reflective of the bus
type than the device or driver.

Use a fixed latency interval for network devices and the
adaptive underrun approach for USB devices - regardless
of driver or device type.

The GPMC based transport on the E100 appears unaffected
by either latency scheme, which defaults to network.

Signed-off-by: Thomas Tsou <ttsou@vt.edu>

git-svn-id: http://wush.net/svn/range/software/public/openbts/trunk@2677 19bc5d8c-e614-43d4-8b26-e1612bc8e597

Transceiver52M/USRPDevice.h

@@ -126,6 +126,9 @@ private:
   /** Set priority not supported */
   void setPriority() { return; }
 
+  /** Only USB bus supported */
+  busType getBus() { return USB; }
+
   /**
 	Read samples from the USRP.
 	@param buf preallocated buf to contain read result