iio: imu: adis16400: fix memory leak

In adis_update_scan_mode_burst, if adis->buffer allocation fails release
the adis->xfer.

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>