msm: synx: handle callback deregistration failure

In case of callback deregistration failure, end further
signaling and memory free for the external sync obj.
Without the change, the memory shared with the external
callback would be released even on deregistration failure,
leading to possible double free, as callback might already
been dispatched.

Change-Id: Ic2e755b0b039a9b7ad2e25abb23949d673bda685
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>