macio: set res_count value to 0 after non-block ATAPI DMA transfers

res_count should be set to the number of outstanding bytes after a DBDMA
request. Unfortunately this wasn't being set to zero by the non-block
transfer codepath meaning drivers that checked the descriptor result for
such requests (e.g reading the CDROM TOC) would assume from a non-zero result
that the transfer had failed.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
