staging: comedi: dt9812: cleanup analog out subdevice (*insn_read)

For aesthetic reasons, rename the function to help with grepping and
rename some of the local vars.

This function returns the current analog output value for the channel
that is cached in the private data. Absorb dt9812_analog_out_shadow()
into this function to make it more concise.

The comedi core expects the (*insn_read) functions to return either
an errno or the number of samples read. Change the final return to
insn->n to make this clearer.

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>