arm/at91: Don't disable irqs in set_next_event and set_mode callbacks

on AT91 the timer irq is shared, so the handler might be entered without
irqs being disabled.  Though this should not happen as the timer irq is
registered early, there have been some reports on the mailing list.

To make debugging that problem easier next time it pops up a
WARN_ON_ONCE is added to the handler if irqs are not off.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>