fifo8: introduce head variable for fifo8_peekpop_bufptr()

fifo8: introduce head variable for fifo8_peekpop_bufptr()

Rather than operate on fifo->head directly, introduce a new head
variable which is set to the value of fifo->head and use it instead.
This is to allow future adjustment of the head position within the
internal FIFO buffer.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Octavian Purdila <tavip@google.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240828122258.928947-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>