staging:iio:ad7291: add missing .driver_module to struct iio_info

Add missing .driver_module of struct iio_info. This prevents the
module from being removed from underneath its users.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>