staging: comedi: pcl818: fix PCL818_FI_DATAHI

The A/D FIFO uses two registers to get each analog data sample.
PCL818_FI_DATALO is the LSB of the data and PCL818_FI_DATAHI is
the MSB of the data. The current define for PCL818_FI_DATAHI is
incorrect and results in the LSB getting read twice.

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>