staging: comedi: jr3_pci: separate out poll state enum

The type of the `state` member of `struct jr3_pci_subdev_private` is
defined in-situ as an enumerated type without a tag.  For aesthetic
reasons, define the type as `enum jr3_pci_poll_state` outside the
containing `struct`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>