ARM: 6460/1: ixp2000: fix type of ixp2000_timer_interrupt

This fixes a build warning existing since at least 2.6.30:

	arch/arm/mach-ixp2000/core.c:217: warning: initialization from incompatible pointer type

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>