staging: comedi: skel: remove thisboard macro

This driver's `thisboard` macro relies on a local variable having a
specific name.  Replace it with a local variable of the same name in the
functions that use it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>