staging:iio:dac:ad5638: Fix channel address

Commit c6fc8062 ("staging:iio: ABI rework - add in_ or out_ prefix to
channnels") added the AD5868_CHANNEL macro to simplify channel initialization.
Unfortunately the macro hardcodes the channel's address to AD5686_ADDR_DAC0. As
a result writing to any of the channels will change the value of the first
channel.

This patch fixes the issue by calculating the channel address based on the
channel number.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>