pldm_visualise_pdrs: check for tty

Check for a tty before using a carriage return.  This yields sane
behavior when redirecting output to something other than a shell e.g.
more, less, etc...

Change-Id: I250fefc531a2aaced9bee433388e25fccb4952ce
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/tools/visualize-pdr/pldm_visualise_pdrs.py b/tools/visualize-pdr/pldm_visualise_pdrs.py
index 442d607..361f433 100755
--- a/tools/visualize-pdr/pldm_visualise_pdrs.py
+++ b/tools/visualize-pdr/pldm_visualise_pdrs.py
@@ -362,8 +362,10 @@
     fru_record_set_pdr = {}
     tl_pdr = {}
     for handle_number, my_dic in get_pdrs(executor):
-        sys.stdout.write("Fetching PDR's from BMC : %8d\r" % (handle_number))
-        sys.stdout.flush()
+        if sys.stdout.isatty():
+            sys.stdout.write(
+                "Fetching PDR's from BMC : %8d\r" % (handle_number))
+            sys.stdout.flush()
         if my_dic["PDRType"] == "Entity Association PDR":
             entity_association_pdr[handle_number] = my_dic
         if my_dic["PDRType"] == "State Sensor PDR":