staging: comedi: c6xdigio: remove C6XDIGIO_SIZE

This define is only used in the comedi_request_region() call and does
not add any additional clarity to the code. Remove it.

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>