util/fifo8: clear fifo head upon reset

util/fifo8: clear fifo head upon reset

To improve the predictability of fifo8_pop_buf(), the fifo head is set
to the start of data buffer upon a reset so that the first call to the
function will be able to retrieve all data in the fifo.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>