staging: comedi: adl_pci9111: cleanup pci9111_ao_insn_read()

Remove the unnecessary comments.

The readback value does not need to be masked.

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>