clockevents: Do not suspend/resume if unused

There is no point in calling suspend/resume for unused clockevents as
they are already stopped and disabled.

This is really important for AT91 as the hardware is a trainwreck and
takes ages to synchronize.

Reported-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1421399151-26800-1-git-send-email-alexandre.belloni@free-electrons.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>