MIPS: Fix sparse warning in incompatiable argument type of clear_user.

The type of the second argument of access_ok should be (void __user *).
The unnecessary conversion of the clear_user address argument was causing
sparse to emit warnings on the __chk_user_ptr check.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>