esp.c: replace esp_fifo_pop_buf() with esp_fifo8_pop_buf() in do_message_phase()

esp.c: replace esp_fifo_pop_buf() with esp_fifo8_pop_buf() in do_message_phase()

The aim is to restrict the esp_fifo_*() functions so that they only operate on
the hardware FIFO. When reading from cmdfifo in do_message_phase() use the
underlying esp_fifo8_pop_buf() function directly.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240324191707.623175-4-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>