staging: comedi: ni_labpc: move labpc_driver and labpc_pci_table

Move the `labpc_driver` and `labpc_pci_table` variables nearer the
module initialization code near the bottom of the module for aesthetic
reasons and to avoid a forward declaration of `labpc_attach()`.

As a consequence, to avoid having to add a forward declaration of the
`labpc_driver` variable, change code that uses
`labpc_driver.driver_name` to use `DRV_NAME` instead, and change code
that uses `labpc_driver.num_names` to use `ARRAY_SIZE(labpc_boards)`
instead.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>