staging:iio:dac:ad5791: Add id entry for the ad5790

The ad5790 has a binary compatible interface to ad5791, so we just have to add
an entry to the drivers device table to add support for it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>