power_supply: twl4030_madc: Check return value of power_supply_register

The return value of power_supply_register() call was not checked and
even on error probe() function returned 0. If registering failed then
during unbind the driver tried to unregister power supply which was not
actually registered.

This could lead to memory corruption because power_supply_unregister()
unconditionally cleans up given power supply.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Fixes: da0a00eb ("power: Add twl4030_madc battery driver.")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>