power_supply: max77693: 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 when getting 'online' and 'present'
   proprties 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>