iio: ad5770r: make devicetree property reading consistent

The bindings file for this driver is defining the property as 'reg' but
the driver was reading it with the 'num' name. The bindings actually had
the 'num' property when added in
commit ea52c212 ("dt-bindings: iio: dac: Add docs for AD5770R DAC")
and then changed it to 'reg' in
commit 2cf3818f ("dt-bindings: iio: dac: AD5570R fix bindings errors").
However, both these commits landed in v5.7 so the assumption is
that either 'num' is not being used or if it is, the validations were not
done.

Anyways, if someone comes back yelling about this, we might just support
both of the properties in the future. Not ideal, but that's life...

Fixes: 2cf3818f ("dt-bindings: iio: dac: AD5570R fix bindings errors")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210818080525.62790-1-nuno.sa@analog.com


Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>