esp.c: introduce esp_fifo_push_buf() function for pushing to the FIFO

esp.c: introduce esp_fifo_push_buf() function for pushing to the FIFO

Instead of pushing data into the FIFO directly with fifo8_push_all(), add a new
esp_fifo_push_buf() function and use it accordingly.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20240324191707.623175-10-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>