power: supply: core: Initialize struct to zero

As we rely on pointers in the battery info to be zero-initialized
such as in the helper function power_supply_supports_vbat2ri()
we certainly need to allocate the struct power_supply_battery_info
with kzalloc() as well. Else this happens:

Unable to handle kernel paging request at virtual address 00280000
(...)
PC is at power_supply_vbat2ri+0x50/0x12c
LR is at ab8500_fg_battery_resistance+0x34/0x108

Fixes: e9e7d165 ("power: supply: Support VBAT-to-Ri lookup tables")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>