firewire: fw-ohci: unify printk prefixes

The messages which can be enabled by fw-ohci's debug module parameter
are changed from KERN_DEBUG to KERN_NOTICE level and uniformly prefixed
with "firewire_ohci: ".  This further simplifies communication with
users when we ask them to capture debug messages.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>