Input: remove race when instantiating polled device attributes

Polled device's attributes controlling polling rate and whether polling is
enabled are attached to input device. We should have device core
instantiate them for us, so that they are created by the time new device
notification is sent to userspace, instead of doing it ourselves
afterwards.

Tested-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>