power: ltc2941-battery-gauge: Fix typo in conversion formula (58 instead of 85)

The driver reported 30% less than actually measured. This turned out to
be caused by a simple typo in the formula to calculate the LSB quantity.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Sebastian Reichel <sre@kernel.org>