iio: dac: build ad5758 driver when AD5758 is selected

Commit 28d1a7ac ("iio: dac: Add AD5758 support") adds the config AD5758
and the corresponding driver ad5758.c. In the Makefile, the ad5758 driver
is however included when AD5755 is selected, not when AD5758 is selected.

Probably, this was simply a mistake that happened by copy-and-paste and
forgetting to adjust the actual line. Surprisingly, no one has ever noticed
that this driver is actually only included when AD5755 is selected and that
the config AD5758 has actually no effect on the build.

Fixes: 28d1a7ac

 ("iio: dac: Add AD5758 support")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20230508040208.12033-1-lukas.bulwahn@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>