Fix mental error in get_hostapd_mac

It appears this code wasn't tested before it was committed,
since it never had a chance... :-)

BUG=None
TEST=Make sure the error:
   AttributeError: 'str' object has no attribute 'Interface'
doesn't happen any more due to
   ap_interface = interface.Interface(interface, self.host)

Change-Id: I42aeedf3e47de64260a84bbb2386dff1f76f22b0
Reviewed-on: https://chromium-review.googlesource.com/186580
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/server/site_linux_router.py b/server/site_linux_router.py
index 31fc08b..d97e033 100644
--- a/server/site_linux_router.py
+++ b/server/site_linux_router.py
@@ -519,8 +519,8 @@
         @return string MAC address like 00:11:22:33:44:55.
 
         """
-        interface = self.get_hostapd_interface(ap_num)
-        ap_interface = interface.Interface(interface, self.host)
+        interface_name = self.get_hostapd_interface(ap_num)
+        ap_interface = interface.Interface(interface_name, self.host)
         return ap_interface.mac_address