parisc: regs_return_value() should return gpr28

While working on kretprobes for PA-RISC I was wondering while the
kprobes sanity test always fails on kretprobes. This is caused by
returning gpr20 instead of gpr28.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # 4.14+