power: supply: z2_battery: Fix possible memleak in z2_batt_probe()

If devm_gpiod_get_optional() returns error, the charger should be
freed before z2_batt_probe returns according to the context. We
fix it by just gotoing to 'err' branch.

Fixes: a3b4388e ("power: supply: z2_battery: Convert to GPIO descriptors")
Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>