staging: comedi: ni_at_a2150: 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: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>