power: supply: charger-manager: Fix a NULL pointer dereference in 'charger_manager_probe()'

'devm_kzalloc()' can return NULL. Return -ENOMEM in this case in order to
avoid a NULL pointer dereference later on.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>