MIPS: Fix syscall tracing interface

Fix pointer computation for stack-based arguments.

Signed-off-by: Lars Persson <larper@axis.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6620/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>