iio:light:rpr0521: Fix timestamp alignment and prevent data leak.

One of a class of bugs pointed out by Lars in a recent review.
iio_push_to_buffers_with_timestamp() assumes the buffer used is aligned
to the size of the timestamp (8 bytes).  This is not guaranteed in
this driver which uses an array of smaller elements on the stack.
As Lars also noted this anti pattern can involve a leak of data to
userspace and that indeed can happen here.  We close both issues by
moving to a suitable structure in the iio_priv().
This data is allocated with kzalloc() so no data can leak apart
from previous readings and in this case the status byte from the device.

The forced alignment of ts is not necessary in this case but it
potentially makes the code less fragile.

>From personal communications with Mikko:

We could probably split the reading of the int register, but it
would mean a significant performance cost of 20 i2c clock cycles.

Fixes: e12ffd24

 ("iio: light: rpr0521 triggered buffer")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Mikko Koivunen <mikko.koivunen@fi.rohmeurope.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200920112742.170751-2-jic23@kernel.org