iio: adc: ti_am335x_adc: Gather the checks on the delays

Move the checks over the delays provided in the device tree to the
location where these values are read to clarify where they come from.

There are no functional changes besides the device structure used to
display the warnings: let's use the ADC instead of the MFD device.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-43-miquel.raynal@bootlin.com