Rename TI_BUFSZ to ESP_FIFO_SZ since this constant is really describing the size
of the FIFO and is not directly related to the TI size.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-39-mark.cave-ayland@ilande.co.uk>