iio: imu: adis16400: release allocated memory on failure

In adis_update_scan_mode, if allocation for adis->buffer fails,
previously allocated adis->xfer needs to be released.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>