Bluetooth: hidp: Initialize list header of hidp session user

When new hidp session is created, list header in l2cap_user is
not initialized and this causes list_empty() to fail in
l2cap_register_user() even if l2cap_user list is empty.

Signed-off-by: Tedd Ho-Jeong An <tedd.an@intel.com>
Tested-by: Jörg Otte <jrg.otte@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>