iio: Use ktime_get_real_ns()

No idea why iio needs wall clock based time stamps, but we can avoid
the timespec conversion dance by using the new interfaces.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>