net/sonic: Clean up and modernize log messages

Add missing printk severity levels by adopting pr_foo() calls for the
platform_driver and dev_foo() calls for the nubus_driver.
Avoid KERN_CONT usage as per advice from checkpatch.
Avoid #ifdef around printk calls.
Don't log driver probe messages after calling register_netdev().

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Chris Zankel <chris@zankel.net>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>