qcacld-3.0: Validate context before bus bw timer start

In SSR case, as a part of hdd_wlan_shutdown, the bus
bw timer is stopped. But pending message of association
completion in the scheduler thread is processed, due to
which the bw timer is started again.
So in shutdown path, bus bw timer is attempted to de-init,
there is a BUG_ON as timer is again started while processing
association completion.

Validate the context in bus bw timer start to avoid bus bw timer
start during SSR case.

Change-Id: I2cce729d7c27732cbd9f14e6953078679d46e8ed
CRs-Fixed: 2495860