m68k/mac: IOP - Modernize printing of kernel messages

  - Introduce helpers for printing debug messages, incl. dummies for
    validating format strings when debugging is disabled,
  - Convert from printk() to pr_*(),
  - Add missing continuations,
  - Drop superfluous casts.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
[FT: Renamed pr_iop* macros, adjusted log message severity and eliminated DEBUG_IOP]
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>