HID: hidp: Add a comment that some devices depend on the current behavior of uniq

Add a comment noting that some devices depend on the destination address being
stored in uniq.

Signed-off-by: Frank Praznik <frank.praznik@oh.rr.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>