staging:iio:dac:ad5064: Prepare driver for the addition of chip variants

Prepare the driver for the addition of chip variants with a different number of
channels. This is done by not hard-coding the number of channels, but instead
add a field to the chip info struct holding the number of channels. Also do not
embed the channel specs into the chip info, but rather store them independently.
This allows sharing the same channel spec between different chip infos.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>