net: rmnet_data: add cleanup in rmnet_config_exit

Add a call to unregister_netdevice_notifier in rmnet_config_exit,
and fix some compilation warnings.

CRs-Fixed: 633585
Change-Id: I0e61c5460b927c3348f4e9815bbd9f842488f14d
Acked-by: Sivan Reinstein <sivanr@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>