tracing/ftrace: fix a bug when switch current tracer to sched tracer

Impact: fix boot tracer + sched tracer coupling bug

Fix a bug that made the sched_switch tracer unable to run
if set as the current_tracer after the boot tracer.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>