goldfish_battery: Use resource_size()

This uses the resource_size() function instead of explicit computation.

Signed-off-by: Silviu-Mihai Popescu <silviupopescu1990@gmail.com>
Signed-off-by: Andrei Epure <epure.andrei@gmail.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>