power: bq24257: Simplify bq24257_power_supply_init()

Eliminate a few lines of code by using the PTR_ERR_OR_ZERO() macro.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>