staging: comedi: ni_labpc_isadma: convert 'dma_buffer_size' to a define

For aesthetics, convert this statis const global variable to a define.

Cleanup the maximum 'size' calc in labpc_suggest_transfer_size(). The modulo
operation will always result in '0'.

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>