power: supply: ltc4162-l: Constify static struct attribute_group

The only usage of it is to put its address in an array of pointers to
const static structs. Make it const to allow the compiler to put it in
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>