sata_promise: ATAPI cleanup

Here's a cleanup for yesterday's sata_promise ATAPI patch:
- add and use a symbolic constant for the altstatus register
- check return status from ata_busy_wait()
- add missing newline in a warning printk()
- update comment in pdc_issue_atapi_pkt_cmd() to clarify
  that the maybe-wait-for-INT issue cannot occur in the
  current driver, but may occur if the driver starts issuing
  ATAPI non-DMA commands as PDC packets

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Jeff Garzik <jeff@garzik.org>