iio: gts-helpers: fix integration time units

The IIO ABI mandates expressing integration times in seconds. The GTS
helper errorneously uses micro seconds in integration_times_available.
Fix this by converting the lists to IIO_VAL_INT_PLUS_MICRO.

Fixes: 38416c28 ("iio: light: Add gain-time-scale helpers")
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/eeacd192c259e885850b5a2dd8b776bccfc44fa8.1681722914.git.mazziesaccount@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>