staging: comedi: ni_65xx: remove 'dio_direction' from private data

The IO Select registers are readable. Remove the need for the
'dio_direction' member in the private data by just checking the
register value for the INSN_CONFIG_DIO_QUERY instruction.

Also, refactor the switch statement to return -EINVAL for unhandled
instructions and have the (*insn_config) return insn->n normally.

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>