qseecom: use scm_call2 for shutdown app

Unlike scm_call2, __qseecom_scm_call2_locked() will unlock
app_access_lock to let another qseecom thread work. But this
mutex should not be released for shutdown app thread to
prevent another thread accessing the resource that is freed
by shutdown thread. So change to use scm_call2 for this cmd.

Change-Id: Ifdd526e0b401cab5c9d6df71d5854022e0f273f0
Signed-off-by: Zhen Kong <zkong@codeaurora.org>