macio: switch pmac_dma_write() over to new offset/len implementation

In particular, this fixes a bug whereby chains of overlapping head/tail chains
would incorrectly write over each other's remainder cache. This is the access
pattern used by OS X/Darwin and fixes an issue with a corrupt Darwin
installation in my local tests.

While we are here, rename the DBDMA_io struct property remainder to
head_remainder for clarification.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1433455177-21243-3-git-send-email-mark.cave-ayland@ilande.co.uk
Signed-off-by: John Snow <jsnow@redhat.com>
