power: supply: sbs-battery: fix CAPACITY_MODE bit naming

"Battery mode" is the name of the register, the bit manipulated by this
code is "CAPACITY_MODE" (Smart Battery System Specifications).

Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>