powerpc: Fix off-by-one error in prom_init.c

This was preventing us from recognizing that we did actually
instantiate RTAS successfully on pSeries.
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/prom_init.c

@@ -268,7 +268,7 @@ static int __init call_prom_ret(const char *service, int nargs, int nret,
 
 	if (rets != NULL)
 		for (i = 1; i < nret; ++i)
-			rets[i] = args.args[nargs+i];
+			rets[i-1] = args.args[nargs+i];
 
 	return (nret > 0) ? args.args[nargs] : 0;
 }