macintosh/adb: Properly mark continued kernel messages

Use pr_cont where appropriate, and switch to pr_foo throughout.
Additionally, lower messages in adb_probe_task to debug level.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
[mpe: Clean up whitespace slightly]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>