PCI: epf-mhi: Simulate async read/write using iATU

Even though iATU only supports synchronous read/write, the MHI stack may
call async read/write callbacks without knowing the limitations of the
controller driver. So in order to maintain compatibility, let's simulate
async read/write operation with iATU by invoking the completion callback
after memcpy.

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>