qcacld-3.0: Fix bus bw timer leak in SSR cases

During SRR case, use hdd_bus_bw_compute_timer_stop in
hdd_wlan_shutdown instead of hdd_bus_bandwidth_destroy
as hdd_bus_bandwidth_destroy must be called only once
at hdd_wlan_exit.

Change-Id: I43813baffb2a3081c4470d1844f266ea0cbfd553
CRs-Fixed: 2289565