parisc/ftrace: set function trace function

With DYNAMIC_FTRACE, we need to implement ftrace_update_trace_func
and not call ftrace_trace_function() directly, as ftrace doesn't
expect calls to this function during code patching.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>