staging: comedi: dt3000: remove unneeded variable

Coccinelle (using scripts/coccinelle/tests/doublebitand.cocci) found this
assignment because 0 is or'd twice.  Upon inspection, I found that the
variable "mode" is only assigned once and used once, and thus can be
cleanly removed.  This patch plugs its value straight into writew() and
then removes the variable.

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>