mfd: Fix ab8500-gpadc to measure charger current

The GPADC in the AB8500 was incorrectly configured when a charger
current channel was selected.

Signed-off-by: Karl Komierowski <karl.komierowski@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>