esp: use pdma_origin directly in esp_pdma_read()/esp_pdma_write()

This is the first step in removing get_pdma_buf() from esp.c.

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: <20210304221103.6369-17-mark.cave-ayland@ilande.co.uk>