staging: comedi: ni_mio_common: fix ni_ao_insn_read()

The comedi core expects (*insn_read) functions to return insn->n
data smaples. Fix this function to work like the core expects.

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>