iio:ad5064: Fix address of the second channel for ad5065/ad5045/ad5025

The ad5065, ad5045, ad5025 use address '3' for the second channel, so they need
their own channel spec.

Note that ad5064_sync_powerdown_mode() also needs to be slightly updated since
it was relying on the fact that chan->address always equaled chan->channel.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>