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>