parisc/kernel/ftrace: Remove function callback casts

In an effort to enable -Wcast-function-type in the top-level Makefile to
support Control Flow Integrity builds, remove all the function callback
casts.
Co-developed-by: Helge Deller <deller@gmx.de>
Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/kernel/ftrace.c

@@ -64,7 +64,8 @@ void notrace __hot ftrace_function_trampoline(unsigned long parent,
 				function_trace_op, regs);
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
-	if (ftrace_graph_return != (trace_func_graph_ret_t) ftrace_stub ||
+	if (dereference_function_descriptor(ftrace_graph_return) !=
+	    dereference_function_descriptor(ftrace_stub) ||
 	    ftrace_graph_entry != ftrace_graph_entry_stub) {
 		unsigned long *parent_rp;