mfd: 88pm800: Fix NULL pointer dereference

Move "device_800_init" to fix NULL pointer error when
calling "device_gpadc_init" as it needs "subchip->regmap_gpadc"
to set registers via regmap interface

Signed-off-by: Yi Zhang <yizhang@marvell.com>
Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>