iio: adis: Remove startup_delay

All timeouts are now handled by a dedicated timeout struct. This
variable is no longer needed.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>