iio: dac: ad7303: use regulator get optional to check for ext supply

Previously, the code was using the of_read_property_bool() to check if
an external regulator was provided. However, this is redundant, as it's
more simple/direct to just ask the regulator is provided, via a
`devm_regulator_get_optional()` call.

Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>