HID: uhid: keep legacy definitions at the bottom of uhid.h

Instead of inlining the legacy definitions into the main part of uhid.h,
keep them at the bottom now. This way, the API is much easier to read and
legacy requests can be looked up at a separate place.

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