power: supply: max17042_battery: simplify getting the adapter of a client

We have a dedicated pointer for that, so use it. Much easier to read and
less computation involved.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>