m68k/atari: Modernize printing of kernel messages

  - Convert from printk() to pr_*(),
  - Add missing continuations, to fix user-visible breakage,
  - Drop useless WARNING prefix,
  - Move trailing spaces to start of continuations.

Note that the "Keyboard overrun" message will now only be generated when
the kernel has been compiled for debugging.

Fixes: 4bcc595c ("printk: reinstate KERN_CONT for printing continuation lines")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>