The value of dma_counter is set once at the start of the transfer and remains
the same until the transfer is complete. This allows the check in esp_transfer_data
to be simplified since dma_left will always be non-zero until the transfer is
completed.

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: <20210304221103.6369-13-mark.cave-ayland@ilande.co.uk>