hwmon: Don't access struct mc13783 directly from mc13783-adc

There is a shiny new mc13783 API function that can be used instead.

While at it refactor the code a bit to reduce code duplication a bit.

This removes the last user of <linux/mfd/mc13783-private.h> and so this
include file can go away.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>