power_supply: lp8788-charger: Fix leaked power supply on probe fail

Driver forgot to unregister charger power supply if registering of
battery supply failed in probe(). In such case the memory associated
with power supply leaked.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Fixes: 98a276649358 ("power_supply: Add new lp8788 charger driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>