power_supply: max14577: Properly handle error conditions

Re-work and fix handling of errors when retrieving power supply
properties:

1. Return errno values directly from get_property() instead of
   storing 'unknown' as intval for given property.

2. Handle regmap_read() errors and return errno code. Previously the
   regmap_read() return code was ignored so an uninitialized value from
   the stack could be used for calculating the property.

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