staging: comedi: cb_pcidas64: rename 'thisboard' variables

For aesthetics, rename the 'thisboard' variables to 'board'. That name
is more commonly used for the boardinfo pointer in comedi drivers.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: NIan Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>