hw/arm_timer.c: Fix bounds check for Integrator timer accesses

There are only three counter/timers on the integrator board:
correct the bounds check to avoid an array overrun. (Spotted
by Coverity, see bug 887883).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
