qseecom: change return value to 0 if app id exists when loading app

Currently qseecom will return the app_id (>0) instead of 0 if the
app id already exists when loading app. Then the userspace client
will release qseecom handle if an non-zero return value is obtained.
Make a change to let qseecom return 0 when app id exist so as to
avoid the incorrect qseecom release.

Change-Id: I03b0b0ae95b2e043fda972aa3b613e7df585154a
Signed-off-by: Zhen Kong <zkong@codeaurora.org>