sh: Don't NOTIFY_STOP for non-UBC breakpoint traps.

Presently the hw_breakpoint code is the primary notifier dispatch for
breakpoint traps, although it's only UBC traps that are of particular
interest there. This patches in a check to allow non-UBC generated
breakpoints to pass through down the remainder of the notifier chain,
giving things like kgdb a chance at getting notified.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>