Correct member type for struct MTSlotInfo

As pointed out by djkurtz@ in a related code review here
https://gerrit.chromium.org/gerrit/#/c/37987/2/x11-drivers/xf86-input-evdev/files/evdev-2.7.3-Add-SYN_DROPPED-handling.patch

EVIOCGMTSLOTS() takes a struct defined as
struct input_mt_request_layout {
      __u32 code;
      __s32 values[num_slots];
};

MTSlotInfo needs to explicitly make code unsigned 32bit and values array
of signed 32bit.

BUG=chromium-os:37778
TEST=compile.

Change-Id: If32ffcc7efea5e79062465967c40bd04456b4538
Reviewed-on: https://gerrit.chromium.org/gerrit/40641
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Commit-Queue: Yufeng Shen <miletus@chromium.org>
Tested-by: Yufeng Shen <miletus@chromium.org>