selftests/ftrace: Fix bash specific "==" operator

Since commit a1d6cd88c8 ("selftests/ftrace: event_triggers: wait
longer for test_event_enable") introduced bash specific "=="
comparation operator, that test will fail when we run it on a
posix-shell. `checkbashisms` warned it as below.

possible bashism in ftrace/func_event_triggers.tc line 45 (should be 'b = a'):
        if [ "$e" == $val ]; then

This replaces it with "=".

Fixes: a1d6cd88c8 ("selftests/ftrace: event_triggers: wait longer for test_event_enable")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/ftrace/test.d/ftrace/func_event_triggers.tc

@@ -42,7 +42,7 @@ test_event_enabled() {
 
     while [ $check_times -ne 0 ]; do
 	e=`cat $EVENT_ENABLE`
-	if [ "$e" == $val ]; then
+	if [ "$e" = $val ]; then
 	    return 0
 	fi
 	sleep $SLEEP_TIME