cpuidle/powernv: avoid double irq enable coming out of idle

Since e1689795

 ("cpuidle: Add common time keeping and irq enabling"),
cpuidle drivers are expected to return from ->enter with irqs disabled.

Update the cpuidle-powernv snooze loop to disable irqs before returning.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>