When the through-8259A mode is used for the timer, the call to
set_irq_handler() will register a NULL handler name, resulting in
"IO-APIC-<NULL>" reported.  Fix by calling ioapic_register_intr() as done
for all the other I/O APIC interrupts.

The 64-bit variation calls set_irq_chip_and_handler_name() here
needlessly and should get fixed with the upcoming merge.
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>