autotest: Cleanup capability parsing in site_linux_system

BUG=None
TEST=./run_remote_tests.sh --remote=chromeos2-shelf3-host3 wifi_check11b
passes.

Change-Id: Id3d88dcb0265011b9c1ee0aff3c5f5a5705604a2
Reviewed-on: https://gerrit.chromium.org/gerrit/59131
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
diff --git a/server/site_linux_system.py b/server/site_linux_system.py
index a032027..7b3b2f4 100644
--- a/server/site_linux_system.py
+++ b/server/site_linux_system.py
@@ -151,17 +151,17 @@
 
 
     def get_capabilities(self):
-        caps = []
+        caps = set()
         phymap = self.phys_for_frequency
         if [freq for freq in phymap.iterkeys() if freq > 5000]:
             # The frequencies are expressed in megaherz
-            caps.append(self.CAPABILITY_5GHZ)
+            caps.add(self.CAPABILITY_5GHZ)
         if [freq for freq in phymap.iterkeys() if len(phymap[freq]) > 1]:
-            caps.append(self.CAPABILITY_MULTI_AP_SAME_BAND)
-            caps.append(self.CAPABILITY_MULTI_AP)
+            caps.add(self.CAPABILITY_MULTI_AP_SAME_BAND)
+            caps.add(self.CAPABILITY_MULTI_AP)
         elif len(self.phy_bus_type) > 1:
-            caps.append(self.CAPABILITY_MULTI_AP)
-        return set(caps)
+            caps.add(self.CAPABILITY_MULTI_AP)
+        return caps
 
 
     def start_capture_params(self, params):