staging: comedi: dt9812: pass the comedi_device * to dt9812_analog_out_shadow()

For aesthetic reasons, instead of passing the 'slot' pointer, pass the
comedi_device pointer to this function and rename the local var 'result'
to simply 'ret'.

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