power: qpnp-charger.c: check power supply change calls

Make sure that a power supply is initialized before
calling power_supply_changed on a particular supply.

This fixes a possible bad magic bug when the power
supply framework tries to schedule the work queue.

Change-Id: Ie9d071208af93144a647047cf0c6d3ae9b94f4c2
Signed-off-by: David Keitel <dkeitel@codeaurora.org>