staging: comedi: das16m1: check for subdev_8255_init() failure

Make sure to check if subdev_8255_init() fails and propogate the
error code.

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>