staging: comedi: adv_pci1724: introduce adv_pci1724_insn_write()

The analog output, offset calib, and gain calib subdevices all use similar
(*insn_write) functions. The onyl difference is the 'mode' that is passed
to the set_dac() helper.

Introduce a new (*insn_write) function that can be used by all the subdevices.
Use the s->private member to pass the 'mode' needed by set_dac(). Remove the
subdevice specific (*insn_write) functions.

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>