The timer interrupt had accidentally been marked IRQ_DISABLED since
IRQ_PER_CPU had been OR-ed in, instead of set. This had been working
by accident for quite a while.

Commit c642b839 changed the behaviour of
IRQ_PER_CPU interrupts, which previously weren't checked for IRQ_DISABLED.
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>