power: supply: ab8500: Use dev_err_probe() for IIO channels

The code obtaining the ADC channels is outdated: it is
trying to work around the IIO subsystem not returning
the right -EPROBE_DEFER error code. Fix this up
by using the dev_err_probe() helper so we defer silently
where appropriate and not bail out if the IIO core
returns -EPROBE_DEFER as happens now.

Cc: Marcus Cooper <codekipper@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>