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

Since commit a1d6cd88 ("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: a1d6cd88

 ("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>