Fix bug in getting chain map in sensitivity tests.

This CL fixes a minor bug in getting the attenuator chain map where the
map was getting evaluated even when known.

Test: Done
Bug: None

Change-Id: I6ff7b3b1ed6826a97667c3f6a0c3c8ab2a642249
Signed-off-by: Omar El Ayach <oelayach@google.com>
diff --git a/acts/tests/google/wifi/WifiSensitivityTest.py b/acts/tests/google/wifi/WifiSensitivityTest.py
index 5b5c298..fc64844 100644
--- a/acts/tests/google/wifi/WifiSensitivityTest.py
+++ b/acts/tests/google/wifi/WifiSensitivityTest.py
@@ -404,10 +404,10 @@
                                 num_of_tries=5,
                                 check_connectivity=False)
         self.dut_ip = self.dut.droid.connectivityGetIPv4Addresses('wlan0')[0]
-        self.atten_dut_chain_map.setdefault(
-            testcase_params['channel'],
-            wputils.get_current_atten_dut_chain_map(self.attenuators, self.dut,
-                                                    self.ping_server))
+        if testcase_params['channel'] not in self.atten_dut_chain_map.keys():
+            self.atten_dut_chain_map[testcase_params[
+                'channel']] = wputils.get_current_atten_dut_chain_map(
+                    self.attenuators, self.dut, self.ping_server)
         self.log.info("Current Attenuator-DUT Chain Map: {}".format(
             self.atten_dut_chain_map[testcase_params['channel']]))
         for idx, atten in enumerate(self.attenuators):