openflow_input: use of_ prefix in input files

No change to generated code.
diff --git a/openflow_input/nicira_role b/openflow_input/nicira_role
index caa2c1a..fdb37f0 100644
--- a/openflow_input/nicira_role
+++ b/openflow_input/nicira_role
@@ -33,7 +33,7 @@
    NX_ROLE_SLAVE = 2,
 };
 
-struct ofp_nicira_controller_role_request {
+struct of_nicira_controller_role_request {
     uint8_t version;
     uint8_t type;
     uint16_t length;
@@ -43,7 +43,7 @@
     uint32_t role;         // 0 other, 1 master, 2 slave
 };
 
-struct ofp_nicira_controller_role_reply {
+struct of_nicira_controller_role_reply {
     uint8_t version;
     uint8_t type;
     uint16_t length;