msm: ipa: a2_service: memory leak in tethered path

Fix a memory leak in case that tethered client has no callback
function and data arrives from A2 on the tethered path.

Change-Id: Ie1bfac7015fb34b4f0d77d0e49238c72d6f558f7
CRs-Fixed: 576846
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Gidon Studinski <gidons@codeaurora.org>