power: supply: twl4030-charger: move allocation of iio channel to the beginning

This is in prepraration for EPROBE_DEFER handling because it is quite
likely that geting the (madc) iio channel is deferred more often than
later steps.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>