mfd: Don't set mc13783 ADREFMODE for touch conversions

Setting ADREFMODE is utter nonsense, but that's hard to read out of the
spec.  Strange enough it's possible to read x and y values even when
it's set.  When unset you can get values not only for the axes, but also
for contact resistance which allows the touch driver to report pressure
values.

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