staging: dgrp: implement error handling in dgrp_create_class_sysfs_files()

There is no any error handling in dgrp_create_class_sysfs_files().
The patch adds code to check return values and propagate them to dgrp_init_module().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>