iio: adis: Introduce timeouts structure

The adis library only allows to define a `startup_delay` which for some
devices is enough. However, other devices define different timeouts with
significantly different timings which could lead to devices to not wait
enough time or to wait a lot more than necessary (which is not
efficient). This patch introduces a new timeout struct that must be
passed into `adis_init()`. There are mainly, for now, three timeouts
used. This is also an introductory patch with the goal of refactoring
`adis_initial_startup()`. New driver's (eg: adis16480, adis16460) are
replicating code for the device initial setup. With some changes (being
this the first one) we can pass this to `adis_initial_startup()`.

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>