clocksource/drivers/timer-atmel-pit: Remove uselesss WARN_ON_ONCE

IRQ handlers are running with IRQ disabled for a while, remove wrong
comment and useless test.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>