The tail of the list lives at the position before the head. This is
mathematically noted as:
```
(head-1) mod size.
```
Unfortunately C, does not have a modulus operator, but a remainder
operator (%).
The reminder operation has a different result than the modulus if
(head -1) is a negative number and size is not a power of two.

Adding size to (head-1) allows the code to run with any value of size.

This does not change the current behaviour of the driver, as the size is
always a power of two, but avoid tedious debugging if we ever change its
size.
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20240323-resend-hwtimestamp-v10-1-b08e590d97c7@chromium.orgSigned-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>