msm: ais: reqmgr: Hold device lock while retrieving session info

There is a possibility for destroy session and link to occur
concurrently, if this does we end up using a freed pointer while
linking. Avoid this by increasing the scope of the device lock.

Change-Id: I6f1a8967564b64aec282081b21ebe0daf23d2124
Signed-off-by: E V Ravi <evenka@codeaurora.org>