staging: comedi: pcmmio: tidy up ao_rinsn()

Rename this function so it has namespace associated with the driver.

Move the 'chan' local variable out of the for () loop. The 'chan' is
constant for the comedi_insn and only needs to be fetched once from
the insn->chanspec.

Also, remove the sanity check of the chan. The comedi core will ensure
that the chan is valid for the subdevice before calling this function.

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>