staging: comedi: cb_pcidas: refactor write_calibration_bitstream()

Refactor this function to handle the common code used to select the
calibration device (trimpot or caldac) and latch the data after
sending the bitstream.

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>