mhi: dev: satellite: synchronize subsystem connect and device assert

Subsystem can connect while device removal is in progress. This
can lead to a corner case where a stale work item is pending in
the workqueue after it has either already been flushed or is
never flushed before parent structure gets freed. Synchronize
threads to avoid this scenario and flush workqueues every time.

CRs-Fixed: 2500822b
Change-Id: I9f337e1b00506e70143cbb74f1c75034c56b38be
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>