staging: comedi: dt2801: change function return type to void

cppcheck was complaining that the variable 'stat' is being reassigned
before the old value is used.  Upon inspection, I found that
dt2801_writecmd() cannot fail, always returns 0, and most callers already
do not bother with assigning its return value anyway, so it makes sense to
just change the return type for this function from int to void, and remove
the two assignments to 'stat'.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>