power: qpnp-fg: fix NULL pointer dereference in soft JEITA handler

Add check for valid battery power_supply before using it for
power_supply notifications.

CRs-Fixed: 1045153
Change-Id: Ica8f14c31a915fb4749b6c7294bd26648599604d
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>