bq27x00 - don't report power-supply change so often.

A power_supply_changed should only be reported on significant changes
such as transition between charging and not.  Incremental changes
such as charge increasing should not be reported - that can easily
be polled for.

[notasas@gmail.com: refreshed for Pali Rohár's patches]
Signed-off-by: NeilBrown <neilb@suse.de>