dfc: Fix dead lock in dfc_svc_init

Handle a dead lock during cleanup due to rtnl_lock between the
unregister notifier and the dfc exit thread.

Change-Id: I3e61b29c305aba269a6370853c3115d0ed06bdff
Acked-by: Ning Cai <ncai@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>