Wifi-Hal: Pass the address of the global variable to NL callback

Current implementation passes the address of a local variable to the
netlink control block callback, ending up to go out of scope if the
callback is invoked after the scope for the current function ends
(from where the registration happened), resulting in an invalid
memory access. Address this issue by having the respective variable
defined with a global scope.

Change-Id: Ied10a8f17f20326864f80ac40b0ceac6791f6cd1
CRs-Fixed: 2427930
Bug: 129092516
Test: Manual test