esp: use ti_wptr/ti_rptr to manage the current FIFO position for PDMA

This eliminates the last user of the PDMA-specific pdma_cur variable which can
now be removed.

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: <20210304221103.6369-24-mark.cave-ayland@ilande.co.uk>