esp.c: improve comment in esp_transfer_data()

esp.c: improve comment in esp_transfer_data()

Whilst working on the previous patch, the existing comment was not enough to
document when the TI command codepath was being used. Update and improve the
comment accordingly.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250711204636.542964-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>