mhi: MHI channel must be unprepared before driver data is nullified

There can be a race condition between driver data is nullified vs in progress flight transfer.
When a driver is removed, there might be some ongoing transfer.
Call mhi_unprepare_from_transfer() to make sure that MHI channel is reset and no more pending transfers remains.

Change-Id: I4ff91530cdbe16452bf6302e831e8d88583d72fa
Signed-off-by: Tri Dung Le <quic_tridle@quicinc.com>