msm: mhi_dev: Keeping event req memory without clearing in close channel

Client initiated close channel is happening and transfer completion
callback is not received within the timeout period. When completion
callback is received it is becoming invalid as the memory is cleared
as part of close channel previously. Here, keeping event req memory
without clearing in close channel API and marking the elements in
flush event request buffer as stale.

Change-Id: I1e15cf51f0819252f79493d8f806352a12801ef6
Signed-off-by: Sai Chaitanya Kaveti <quic_skaveti@quicinc.com>