bus: mhi: ep: Add mhi_cntrl->flush_async() callback to flush the async read/write

MHI EP stack makes use of the MHI controller drivers like MHI EPF to do
read/write to the host memory. And that driver is free to use mechanisms
like DMA to offload the read/write operations.

So if DMA is used for offload, then there is no guarantee that those DMA
operations would be completed by the time mhi_ep_remove() gets called. This
can lead to UAF (Use-After-Free) issues as the DMA callback can trigger
xfer_cb() even after mhi_ep_remove() has returned.

So to fix this issue, introduce the mhi_cntrl->flush_async() callback and
call it in mhi_ep_remove() to drain all the in-flight async transfers
before disconnecting the channels.

The completion handlers keep triggering xfer_cb() as long as it is set. So
flushing the transfers after notifying the client about the disconnect
(-ENOTCONN) would still let a success callback slip through afterwards and
lead to the same UAF. So disable the channels first to prevent new
transfers, then flush the in-flight transfers so that their completions are
delivered while xfer_cb() is still valid and only then notify the
disconnect and clear xfer_cb().

Fixes: 2547beb00ddb ("bus: mhi: ep: Add support for async DMA read operation")
Fixes: ee08acb58fe4 ("bus: mhi: ep: Add support for async DMA write operation")
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Cc: stable+noautosel@kernel.org # Needs dmaengine driver fix as well
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>

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