qcacld-3.0: Refactor hdd callback (de)registration into a single function

As part of "load once unload never" architecture, cds modules can
be opened and closed several times. Refactor the hdd callback registration/
de-registration into hdd_register_cb/hdd_deregister_cb which can
be invoked during the open/close of the modules respectively.

Change-Id: I2da9018ff2fa6188e61e007c896bc13e8fddcb7d
CRs-Fixed: 1026817
diff --git a/core/hdd/src/wlan_hdd_tsf.c b/core/hdd/src/wlan_hdd_tsf.c
index e98759a..e230925 100644
--- a/core/hdd/src/wlan_hdd_tsf.c
+++ b/core/hdd/src/wlan_hdd_tsf.c
@@ -201,7 +201,7 @@
  *
  * Return: 0 for success or non-zero negative failure code
  */
-static int hdd_get_tsf_cb(void *pcb_cxt, struct stsf *ptsf)
+int hdd_get_tsf_cb(void *pcb_cxt, struct stsf *ptsf)
 {
 	struct hdd_context_s *hddctx;
 	struct hdd_adapter_s *adapter;