staging: comedi: amplc_pc236: Remove thisboard and devpriv macros

The 'thisboard' and 'devpriv' macros rely on a local variable having a
specific name and yield pointers derived from that local variable.
Replace the macros with local variables wherever they occur.

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