staging: comedi: quatech_daqp_cs: fix bug in daqp_ao_insn_write()

The comedi core expects (*insn_write) functions to write insn->n values
to the hardware and return the number of values written.

Currently, this function only writes the first value. Fix it 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>