Prior to this commit the code was using 1 notifier_block for all
types of charger cable, this is incorrect as the notifier_block
becomes part of a linked-list and now the same notifier_block
is part of 3 linked lists.

This commit fixes this by using a separate nb per extcon cable.

Note this happened to work fine sofar because axp288_charger was the only
listener, so when added to each of the 3 notifier chains, the next pointer
in the nb would be set to 0, so we've 3 heads pointing to the same nb,
with its next pointing to NULL. But as soon as we mix in a second extcon
consumer things will go boom.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>