at91: warn if irqs are enabled in set_next_event

Thomas claims that irqs are disabled when set_next_event is called.  But
David and Remy claim they saw irqs being enabled here.  As both sides
don't seem to have time to investigate here, start with a warning that
might trigger if the problem still exists.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-By: David Brownell <dbrownell@users.sourceforge.net>
Cc: Bill Gatliff <bgat@billgatliff.com>
Acked-By: Remy Bohmer <linux@bohmer.net>
Cc: Thomas Gleixner <tglx@linutronix.de>