MIPS: Prevent compiler warning from cop2_{save,restore}

MIPS: Prevent compiler warning from cop2_{save,restore}

The no-op cases of cop2_save & cop2_restore lead to the following
warnings being emitted during build with recent versions of gcc (tested
using gcc 4.8.3 from the Mentor Sourcery CodeBench 2014.05 toolchain):

  In file included from ./arch/mips/include/asm/switch_to.h:18:0,
                   from kernel/sched/core.c:78:
  kernel/sched/core.c: In function 'finish_task_switch':
  include/asm-generic/current.h:6:45: warning: value computed is not used [-Wunused-value]
   #define get_current() (current_thread_info()->task)
                                               ^
  ./arch/mips/include/asm/cop2.h:48:32: note: in definition of macro 'cop2_restore'
   #define cop2_restore(r)  do { (r); } while (0)
                                  ^
  include/asm-generic/current.h:7:17: note: in expansion of macro 'get_current'
   #define current get_current()
                   ^
  ./arch/mips/include/asm/switch_to.h:114:16: note: in expansion of macro 'current'
     cop2_restore(current);     \
                  ^
  kernel/sched/core.c:2225:2: note: in expansion of macro 'finish_arch_switch'
    finish_arch_switch(prev);
    ^

Avoid the warning by "using" the value by casting to void.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7880/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>