power: supply: sbs-battery: correct capacity mode selection bits

The capacity mode bit is bit 15. Currently it is written as
default initialized enum and never shifted. This leads to
a behaviour where the BATTERY_MODE is not correctly
recognized and set again.

This commit initializes the enum accordingly.

Signed-off-by: Michael Heinemann <committed@heine.so>
Tested-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>