iio: use timespec64 based interfaces for iio_get_time_ns()

We have replacements for all the deprecated timespec based interfaces now,
so this can finally convert iio_get_time_ns() to consistently use the
nanosecond or timespec64 based interfaces instead, avoiding the y2038
overflow.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>