max17042_battery: Bugfix of incorrect voltage register value interpretation

The calculation had error in getting voltage values from
MAX17042 registers. The least bit denotes 78.125uV (625/8).

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Philip Rakity <prakity@marvell.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>