msm: synx: validate external callback

Validate the external callback to prevent acting
on delayed invocations, after the corresponding
handle has been released and reallocated by the
clients.
Change also avoids release of memory shared with
external callback if the callback de-registration
fails. This avoids double free issue if callback
has been queued up for dispatch during the cleanup.

Change-Id: Ib784c8a630a7c45ace946edab34e36ab1aec1869
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>