staging: comedi: rti800: tidy up rti800_ao_insn_read()

For aesthetic reasons, tidy up the parameters passed to this function.

The comedi core expects (*insn_read) functions to return the number
of data values actually read (insn->n). Change the return to make
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>