powerpc/mpc83xx: Use wait_event_freezable() for freezable kthread

powerpc/mpc83xx: Use wait_event_freezable() for freezable kthread

A freezable kernel thread can enter frozen state during freezing by
either calling try_to_freeze() or using wait_event_freezable() and its
variants. So for the following snippet of code in a kernel thread loop:
  wait_event_interruptible();
  try_to_freeze();

We can change it to a simple wait_event_freezable() and then eliminate
a function call.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231221044510.1802429-3-haokexin@gmail.com