Add debug_log() macro and convert the driver to use it.

[ This makes debug messages to be always prefixed with "ide-scsi: "
  and use KERN_INFO level. ]

While at it:

* Change "DMA complete" debug message to "DMA finished" to match
  other ATAPI device drivers.

* Use __func__.

Cc: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>