[MIPS] Fix oprofile module unloading

    
When unloading oprofile module with timer-mode, oprofile_arch_exit
dereferences a NULL pointer.
    
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>