essl_spi_rdbuf_polling() function

- ESP_OK: on success - or other return value from :cpp:func:`spi_device_transmit`.

``out_data`` should be prepared in words and in the DRAM. The buffer may be written in words by the DMA. When a byte is written, the remaining bytes in the same word will also be overwritten, even the ``len`` is shorter than a word.