esp: move PDMA length adjustments into esp_pdma_read()/esp_pdma_write()

Here the updates to async_len and ti_size are moved into the corresponding
esp_pdma_read()/esp_pdma_write() function to eliminate the reference to
pdma_cur in do_dma_pdma_cb().

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-23-mark.cave-ayland@ilande.co.uk>