powerpc/perf: Fix sampling enable for PPC970

The logic to distinguish marked instruction events from ordinary events
on PPC970 and derivatives was flawed.  The result is that instruction
sampling didn't get enabled in the PMU for some marked instruction
events, so they would never trigger.  This fixes it by adding the
appropriate break statements in the switch statement.

Reported-by: David Binderman <dcb314@hotmail.com>
Cc: stable@kernel.org
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>