target-ppc: Altivec's mtvscr Decodes Wrong Register

The Move to Vector Status and Control Register (mtvscr) instruction
uses VRB as the source register.  Fix the code generator to correctly
decode the VRB field.  That is, use "rB(ctx->opcode)" instead of
"rD(ctx->opcode)".
Signed-off-by: NTom Musta <tommusta@gmail.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>

target-ppc/translate.c

@@ -6848,7 +6848,7 @@ static void gen_mtvscr(DisasContext *ctx)
         gen_exception(ctx, POWERPC_EXCP_VPU);
         return;
     }
-    p = gen_avr_ptr(rD(ctx->opcode));
+    p = gen_avr_ptr(rB(ctx->opcode));
     gen_helper_mtvscr(cpu_env, p);
     tcg_temp_free_ptr(p);
 }