staging: comedi: quatech_daqp_cs: fix daqp_ao_insn_write()

The (*insn_write) functions are expected to write 'insn->n' number
of samples to the channel. Fix this function so it works as the
comedi core expects.

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>