target-openrisc: Clean up triple QOM casts

Instead of calling openrisc_env_get_cpu(), casting to CPU() via the
ENV_GET_CPU() compatibility macro and casting back to OPENRISC_CPU(),
just call openrisc_env_get_cpu() directly.

ENV_GET_CPU() is meant as workaround for target-independent code only.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/target-openrisc/interrupt_helper.c b/target-openrisc/interrupt_helper.c
index 79f5afe..a176441 100644
--- a/target-openrisc/interrupt_helper.c
+++ b/target-openrisc/interrupt_helper.c
@@ -23,7 +23,7 @@
 
 void HELPER(rfe)(CPUOpenRISCState *env)
 {
-    OpenRISCCPU *cpu = OPENRISC_CPU(ENV_GET_CPU(env));
+    OpenRISCCPU *cpu = openrisc_env_get_cpu(env);
 #ifndef CONFIG_USER_ONLY
     int need_flush_tlb = (cpu->env.sr & (SR_SM | SR_IME | SR_DME)) ^
                          (cpu->env.esr & (SR_SM | SR_IME | SR_DME));