staging: comedi: pcmmio: remove 'subpriv' macro

This macro relies on a local variable having a specific name. Remove
the macro and add a local variable where used.

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>