power: supply: axp288_charger: Fix missing mutex_init()

The driver allocates the mutex but not initialize it.
Use mutex_init() on it to initialize it correctly.

Fixes: ed229454 ("power: supply: axp288-charger: Optimize register reading method")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>