staging: comedi: dmm32at: define the FIFO Depth register

This driver currently does not use the FIFO. For completeness, define
the offset to the FIFO Depth register.

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>