staging: comedi: ni_at_ao: remove ATAO_SIZE define

This define is only used during the board attach to specify the
I/O region length. Remove the define 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>