MIPS: APRP: Unregister rtlx interrupt hook at module exit

MIPS: APRP: Unregister rtlx interrupt hook at module exit

If the aprp_hook is not assigned back to NULL, it will still be called
after module exits. This is not wanted.

Reviewed-by: Steven J. Hill <Steven.Hill@imgtec.com>
Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: john@phrozen.org
Patchwork: https://patchwork.linux-mips.org/patch/6590/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>