pci: msm: Flush workqueue and destroy it in mhi controller unregister

In mhi_unregister_mhi_controller function, flush the work queue and then
destroy it for releasing the memory of unused workqueues.
When multiple mhi-based WLAN endpoints are attached directly or
over switch, then there can be multiple mhi controllers and
so CNSS driver calls for mhi controller registration multiple times.
Each time invoking mhi controller register creates another set of
workqueues and previous workqueues become stale.
By doing so, it will consume the system memory.

So, better to release the memory assigned to workqueue by destroying
the work queue in mhi_unregister_mhi_controller function.

Change-Id: I838371e9c00969a64e658e6175115363ccb916bf
Signed-off-by: Jyothi Kumar Seerapu <quic_jseerapu@quicinc.com>