slim-msm-ngd:Avoid using stale pipe handle

Make sure to use pipe handle from the TX
endpoint rather than using a stored handle.
The stored handle may become invalid if SSR
happens between storing of that handle
and locking the mutex.

Change-Id: I53e8f860c2d50061b6c4e6ce2f944c1e2ed8dff2
Signed-off-by: Dilip Kota <dkota@codeaurora.org>