mfd: Fixed unconditional reset of the mc13xxx ADC reading enable bits

When the ADC is being prepared for a single or multiple channel reading,
the adc0 register is reconfigured without taking the lithium cell, charge
current and battery current reading enable bits into account. Which results
in clearing the bits.

Signed-off-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>