max17042: Change capacity property to use reported SOC register

The SOC register (0dh) reports the state of charge before empty
compensation adjustments are applied.  The max value reported by this
register will decrease as the battery ages.

Use the RepSOC register (06h) to report the capacity of the
battery. RepSOC contains a filtered version of the battery capacity
after empty compensation adjustments have been applied.

Reported-by: Gary Keyes <gary.e.keyes@intel.com>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>