ab8500_charger: Handle gpadc errors

Gracefully handle gpadc conversion errors.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Reviewed-by: Johan BJORNSTEDT <johan.bjornstedt@stericsson.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>