staging: android: timed_output: fix sysfs file creation race

The sysfs file for the driver was being created _after_ the device was
announced to userspace, causing a race with any tools looking for sysfs
files.

Fix the race by using the default attribute group for the class.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>