msm: mhi_dev: Fix incorrect transfer size in completion event

Set the correct transfer size instead of the TRE size when sending
completion event to host in mhi_dev_transfer_completion_cb. Also,
do not access the MHI transfer request structure after the client
callback has been called since it may be re-used for other transfers.
Copy the required fields locally instead.

Change-Id: I6b2cbec3bb3ebfaead926054077e6c0bca85b90e
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>