Input: do not emit unneeded EV_SYN when suspending

Do not emit EV_SYN/SYN_REPORT on suspend if there were no keys that are
still pressed as we are suspending the device (and in all other cases when
input core is forcibly releasing keys via input_dev_release_keys() call).

Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>