iio: adc: max1027: Don't reject external triggers when there is no IRQ

iio: adc: max1027: Don't reject external triggers when there is no IRQ

External triggers do not necessarily need the EOC interrupt to be
populated to work properly. The end of conversion status may either come
from an interrupt or from a sufficient enough extra delay. IRQs are not
mandatory so move the triggered buffer setup out of the IRQ condition
and add the logic to wait enough time for all the requested conversions
to be in the device's FIFO.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20210921115408.66711-17-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>