charger-manager: Ensure event is not used as format string

The exposed interface for cm_notify_event() could result in the event msg
string being parsed as a format string. Make sure it is only used as a
literal string.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Cc: Anton Vorontsov <cbou@mail.ru>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>