max8925_power: Fix incorrect voltage and current calculation

The datasheet indicates a 12 bit value is returned for i2c
registers for voltage and current.  Code was assuming 8 bits.
But default for chip is 12 bit return value.

Voltage is returned in 2mV units  -- adjust to return as uV
per linux power spec

Adjust current calculation to return units in uA.

Signed-off-by: Philip Rakity <prakity@marvell.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>