staging: comedi: das16m1: tidy up misc. defines

For aesthetics, move these after the register map defines and rename
the FIFO_SIZE define.

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>