sparc64: Disable timer interrupts in fixup_irqs().

When a CPU is offlined, we leave the timer interrupts disabled
because fixup_irqs() does not explicitly take care of that case.

Fix this by invoking tick_ops->disable_irq().

Based upon analysis done by Paul E. McKenney.

Signed-off-by: David S. Miller <davem@davemloft.net>