staging: comedi: adv_pci1710: rename 'this_board' local variables

For aesthetics, rename all the 'this_board' local variables to simply
'board'. That's more common in comed drivers.

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>