Commit b632a148b677b773ff155f9de840b37a653567b9 (target-ppc: QOM method
dispatch for MMU fault handling) introduced a use of ENV_GET_CPU()
inside target-ppc/ code. Use ppc_env_get_cpu() instead.
Purely cosmetic, non-functional change to aid in locating and removing
ENV_GET_CPU() usages.
Signed-off-by: Andreas Färber <afaerber@suse.de>