staging: gasket: drop gasket_cdev_get_info, use container_of

Remove gasket_cdev_get_info(), use container_of() directly instead,
drop unnecessary NULL checks.

Reported-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Zhongze Hu <frankhu@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>