power: supply: make device_attribute const

Make these const as they are only passed as an argument to the
function device_create_file and device_remove_file and the corresponding
arguments are of type const.
Done using Coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>