drivers: power: twl4030_charger: fix link problems when building as module

If either twl4030_charger or twl4030_madc is configured as MODULE,
we get build (link) errors.

To solve, the direct call of twl4030_get_madc_conversion() is replaced
by a call to iio_read_channel_processed().

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>