GATT: don't update link status for apps that were never connected

Right now, in GATT_Deregister we make a call to
gatt_update_app_use_link_flag, which might result in a call to
GATT_SetIddleTimeout on links that this client was never connected to.

In order to fix that, we must check if gatt_update_app_hold_link_status
made any changes to p_tcb->app_hold_link. If not, then skip updating
the timer.

Change-Id: I6f18ec7fe4e80e048adc9e360cae703b7d64d62a