staging: comedi: ni_pcidio: remove this_board macro

This macro relies on a local variable having a specific name and
derives a pointer from that local variable.

It's only used in the attach and we already have the local variable
'board' that has the same information. Use that instead.

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