[PATCH] libata ata_data_xfer() fix

PATCH 1/2: ata_data_xfer() fix

Changes:
   - Modify ata_mmio_data_xfer() and ata_pio_data_xfer() to handle odd-lengthed buffer.
   - Add some function comments

This patch does not reuse ap->pad as alignment buffer since
using local variable seems good enough.

Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>