staging: comedi: quatech_daqp_cs: document the 16-bit registers

All the registers are 8-bit except for the analog output and timer
registers. For clarity, add some comments.

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>