iio: light: apds9960: correct ->last_busy count

Add missing pm_runtime_mark_last_busy to apds9960_set_power_state
function.

Unless pm_runtime_mark_last_busy is called the
pm_runtime_put_autosuspend may put the device into suspend before the
delay time requested.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>