fix typo in log output leading to missing variable error

* the change I9305ba791dd43c9088a7bcbcc9af568ae127f60f introduced
  additional logging to allow easier differentiation between tests, but
  also accidentally added the variable "olt_device_id" instead of
  "onu_device_id" in one of those outputs, leading to an error saying:
        Variable '${olt_serial_number}' not found.
* this commit fixes this typo by replacing the variable "olt_device_id"
  in the log output with "onu_device_id"

Change-Id: I3bb10314a2a174459425537ee5fec4510cbea58e
Signed-off-by: Jan Klare <jan.klare@bisdn.de>
diff --git a/tests/functional/Voltha_PODTests.robot b/tests/functional/Voltha_PODTests.robot
index 260fc94..ae4873d 100755
--- a/tests/functional/Voltha_PODTests.robot
+++ b/tests/functional/Voltha_PODTests.robot
@@ -430,7 +430,7 @@
         ...    REACHABLE    ${src['olt']}
         ${rc}    ${output}=    Run and Return Rc and Output
         ...    voltctl -c ${VOLTCTL_CONFIG} device disable ${onu_device_id}
-        Should Be Equal As Integers    ${rc}    0   Could not disable device ${olt_device_id}
+        Should Be Equal As Integers    ${rc}    0   Could not disable device ${onu_device_id}
         Wait Until Keyword Succeeds    ${timeout}    5s
         ...    Validate Device    DISABLED    UNKNOWN
         ...    REACHABLE    ${src['onu']}    onu=false