iio: buffer: fix timestamp alignment when quaternion in scan

iio: buffer: fix timestamp alignment when quaternion in scan

Fix timestamp alignment when a scan buffer contains an element larger
than sizeof(int64_t). Currently s32 quaternions are the only such
element, and the one driver that has this (hid-sensor-rotation) has a
workaround in place already so this change does not affect it.

Previously, we assumed that the timestamp would always be 8-byte aligned
relative to the end of the scan buffer, but in the case of a scan buffer
a 16-byte quaternion vector, scan_bytes == 32, but the timestamp needs
to be placed at offset 16, not 24.

ts_offset is now a value in bytes so we have to change how the array
access is done.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>