Fix compilation with UHD 3.15. Add dependency info to readme.

devices/usrp/deviceusrpparam.cpp

@@ -67,7 +67,7 @@ bool DeviceUSRPParams::open(const char *deviceStr, bool channelNumOnly)
             m_srRangeTx = uhd::meta_range_t(std::min(txLow.start(), txHigh.start()), std::max(txLow.stop(), txHigh.stop()));
 
             // Need to restore automatic clock rate
-            uhd::property_tree::sptr properties = m_dev->get_tree();
+            uhd::property_tree::sptr properties = m_dev->get_device()->get_tree();
             if (properties->exists("/mboards/0/auto_tick_rate"))
             {
                 properties->access<bool>("/mboards/0/auto_tick_rate").set(true);

plugins/samplesource/usrpinput/readme.md

@@ -132,3 +132,10 @@ This label turns green when data is being received from the device.
   - **T**: turns red if stream experiences timeouts
   
 The stream warning indicators are reset when the acqusition is started.
+
+<h2>Dependendices</h2>
+
+On Ubuntu 20, the libuhd-dev package should be installed. The FPGA images then need to be downloaded with:
+
+sudo /usr/lib/uhd/utils/uhd_images_downloader.py
+