staging: comedi: rti802: tidy up rti802_ao_insn_read()

To clarify the function a bit, add a local variable for the 'chan'
that is being read and change the final return to 'insn->n'.

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>