Adds a check of the FORCE_20PCT bit when getting the precharge
current value.

According to the bit description for the FORCE_20PCT bit, when
FORCE_20PCT is true, the precharge current target is 50% of
what is configured in the IPRECHG bit field.
Signed-off-by: Ricardo Rivera-Matos <rriveram@opensource.cirrus.com>
Reviewed-by: David Rhodes <drhodes@opensource.cirrus.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>