[POWERPC] ppc: Add missing calls to set_irq_regs

In the timer_interrupt we were not calling set_irq_regs() and if we are
profiling we will end up calling get_irq_regs().  This causes bad things to
happen.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>