target-ppc: Don't overuse ENV_GET_CPU()

Commit b632a148 (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: NAndreas Färber <afaerber@suse.de>

target-ppc/mmu_helper.c

@@ -2890,7 +2890,7 @@ void helper_booke206_tlbflush(CPUPPCState *env, uint32_t type)
 void tlb_fill(CPUPPCState *env, target_ulong addr, int is_write, int mmu_idx,
               uintptr_t retaddr)
 {
-    CPUState *cpu = ENV_GET_CPU(env);
+    CPUState *cpu = CPU(ppc_env_get_cpu(env));
     PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu);
     int ret;