[nrf fromtree] net: openthread: fix otPlatBleEnable

If BLE has already been enabled, `bt_enable` returns immediately
and does not call passed callback function.
In this case, call `bt_conn_cb_register` to ensure that callbacks
are registered correctly.

Signed-off-by: Maciej Baczmanski <[email protected]>
(cherry picked from commit 1bd8e4459ae161588c5310600dca991bf8d4dbec)

modules/openthread/platform/ble.c

@@ -476,7 +476,7 @@ otError otPlatBleEnable(otInstance *aInstance)
 		LOG_WRN("BLE enable failed with error code %d", err);
 		return OT_ERROR_FAILED;
 	} else if (err == -EALREADY) {
-		err = k_sem_take(&ot_plat_ble_init_semaphore, K_MSEC(500));
+		bt_conn_cb_register(&conn_callbacks);
 		return OT_ERROR_NONE;
 	}