staging: comedi: rtd520: remove thisboard macro

The macro 'thisboard' relies on a local variable having a specific
name and yeilds a pointer derived from that local variable. Replace
the macro with a local variable where used and use the comedi_board()
helper to get the pointer.

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>