iio: ti-ads8688: Update buffer allocation for timestamps

Per Jonathan Cameron, the buffer needs to allocate room for a
64 bit timestamp as well as the channels.  Change the buffer
to allocate this additional space.

Fixes: 2a864877

 ("iio: adc: ti-ads8688: add trigger and buffer support")
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>