wlan: Fix for unregistering IPv4 notifier rightly.

Correct the register and unregister sequence of IPv4
notifier block.

Change-Id: Ib8d77153adb2d71295e2a77a8e228ec55f967f2a
CRs-Fixed: 599742
diff --git a/CORE/HDD/inc/wlan_hdd_main.h b/CORE/HDD/inc/wlan_hdd_main.h
index 1bc3073..5c4cc15 100644
--- a/CORE/HDD/inc/wlan_hdd_main.h
+++ b/CORE/HDD/inc/wlan_hdd_main.h
@@ -840,6 +840,7 @@
     
    /** IPv4 notifier callback for handling ARP offload on change in IP */
    struct notifier_block ipv4_notifier;
+   bool ipv4_notifier_registered;
    struct work_struct  ipv4NotifierWorkQueue;
 
    //TODO Move this to sta Ctx