vt8231: Fix unchecked return status

Check the return status from device_create_file() and also use
the newer and cleaner sysfs creation functions.
Signed-off-by: NRoger Lucas <roger@planbit.co.uk>
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>