staging: comedi: ni_660x: remove enum ni_660x_register_direction

This enum is used to define the, unused, 'direction' of each register
in struct NI_660xRegisterData. Remove the unused member, as well as
the enum.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>