Previously, all the remotes attached to the same receiver would share the
same power_supply. That's not good as the remotes will constantly change
the battery information according to their own state.

To have something generic enough, we introduce struct wacom_battery
which regroups all the information we need for a battery.
Signed-off-by: NBenjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: NPing Cheng <pingc@wacom.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>