staging:iio:dac:ad5446: Do not check for individual chip ids in probe

Use the chip_info's int_vref_mv field to decide whether a certain chip has a
internal reference or not. There is no need to check for individual chip ids.

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>