smb347-charger: Clean up battery attributes

CURRENT_NOW and VOLTAGE_NOW should be instantaneous readings
from power supply(ex: battery).

smb347 charger driver reports charge voltage for VOLTAGE_NOW
and charge current for CURRENT_NOW attributes which are not
instantaneous readings.

This patch removes the battery VOLTAGE_NOW and CURRENT_NOW
properties from the driver and also removes hw_to_current()
which is not required anymore.

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>