staging: comedi: das1800: don't pass struct comedi_cmd by value

Various functions in das1800.c take a `struct comedi_cmd` parameter by
value.  Change them to pass the parameter by reference instead.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>