staging: gasket: apex: fold device add/remove logic inline

Gasket device drivers are now in charge of the device add and remove
sequences; the framework callbacks for these are deleted.  Move the
apex device add callback code to the probe function.  Apex did not
implement the removal callback.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>