staging: comedi: adq12b: tidy up adq12b_ai_rinsn()

For aesthetics, rename this function.

Clean up the local variables by reusing 'val' instead of using
separate variables for the:
  'status' - use once to trigger the first A/D conversion
  'hi' and 'lo' - used to read the A/D conversion result

For aesthetics, change the final return to insn->n and remove the
comment.

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>