mfd: ab8500-gpadc: Use new ab8500_gpadc_get() with name parameter

The new format of ab8500_gpadc_get() accepts a device name as a
parameter to specify which device to retrieve. This patch
enforces the use of that new format.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Philippe Langlais <philippe.langlais@linaro.org>