HID: uhid: use __packed__ for uhid_feature_answer_req

We use __packed__ for all API structures so we can extend them without
breaking alignment rules. We do try to explicitly align the structures,
but to be safe we also use __packed__.
uhid_feature_answer_req is already 64bit aligned so we can add __packed__
without breaking ABI.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>