powerpc/64: Don't call trace_hardirqs_on() in prep_irq_for_idle()

Since commit a01353cf ("cpuidle: Fix ct_idle_*() usage"), the
cpuidle entry code calls trace_hardirqs_on() (actually
trace_hardirqs_on_prepare()) in ct_cpuidle_enter() before calling into
the cpuidle driver.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230406144535.3786008-2-mpe@ellerman.id.au