autotest: sys_power: define pause_ethernet_fd as global

The assignment "pause_ethernet_fd = open..." in do_suspend made python
think it's a local var, and causing:
UnboundLocalError: local variable 'pause_ethernet_fd' referenced before assignment

BUG=chromium:677572
TEST=Run power_SuspendStress.bare on Fizz without error

Change-Id: I4c7c3b859bf44788dac1efc41b0ff0744ceb009b
Signed-off-by: Ben Zhang <benzh@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/820895
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>
diff --git a/client/cros/power/sys_power.py b/client/cros/power/sys_power.py
index a2bfe26..3f4353f 100644
--- a/client/cros/power/sys_power.py
+++ b/client/cros/power/sys_power.py
@@ -144,6 +144,7 @@
     """
 
     # stop check_ethernet.hook from running until the test exits
+    global pause_ethernet_fd
     if pause_ethernet_fd == 0:
         # we don't write to the file - but we might need to create it.
         pause_ethernet_fd = open(PAUSE_ETHERNET_HOOK_FILE,'a+')