staging: comedi: dt282x: remove DT2821_SIZE define

This define is only used in the comedi_request_region() call to specify
the size of the I/O region. Remove it and just open code the value.

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>