Fixed a required type error

Fix error:
**ERROR** Invalid variable type:
valid_values:  [DHCP Address] <unicode>
required_type: list

Signed-off-by: Tony Lee <tony.lee@quantatw.com>
Change-Id: Ib6e04633e5c56acaf916ae22fa4fb8da4349c741
diff --git a/ipmi/test_ipmi_network.robot b/ipmi/test_ipmi_network.robot
index 5e601fc..63ea9e4 100644
--- a/ipmi/test_ipmi_network.robot
+++ b/ipmi/test_ipmi_network.robot
@@ -118,7 +118,7 @@
           Exit For Loop IF  "${ip_address_source}" != 'None'
     END
 
-    Valid Value  lan_config['IP Address Source']  [${ip_address_source}]
+    Valid Value  lan_config['IP Address Source']  ['${ip_address_source}']
 
 
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