staging: gasket: apex return error on sysfs show of missing attribute

Apex sysfs show function return -ENODEV if the attribute is not present,
rather than silently failing from the standpoint of the userspace
accessor.

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