power: supply: max17040: handle device_property_read_u8_array() failure

The device_property_read_u8_array() call should not fail because it is
preceded with device_property_count_u8() and check for number of
readable u8 values.  However the code is more obvious and easier to read
if the return value of device_property_read_u8_array() is checked.
Otherwise reader needs to investigate whether really there is no risk of
using random stack values of 'rcomp' variable.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>