selftests/ftrace: Correctly enable event in instance-event.tc

Function instance_set() expects to enable event 'sched_switch', so we
should set 1 to its 'enable' file.

Testcase passed after this patch:
# ./ftracetest test.d/instances/instance-event.tc
=== Ftrace unit tests ===
[1] Test creation and deletion of trace instances while setting an event
[PASS]

# of passed: 1
# of failed: 0
# of unresolved: 0
# of untested: 0
# of unsupported: 0
# of xfailed: 0
# of undefined(test bug): 0

Signed-off-by: Zheng Yejian <[e-mail]>
Acked-by: Masami Hiramatsu (Google) <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ftrace/test.d/instances/instance-event.tc

···
 
 instance_set() {
         while :; do
-                echo 1 > foo/events/sched/sched_switch
+                echo 1 > foo/events/sched/sched_switch/enable
         done 2> /dev/null
 }