Generic battery handling code is spread between the pen and pad codepaths
since battery usages may appear in reports for either. This makes it
difficult to concisely see the logic involved. Since battery data is
not treated like other data (i.e., we report it through the power_supply
subsystem rather than through the input subsystem), it makes reasonable
sense to split the functionality out into its own functions.

This commit has the generic battery handling duplicate the same pattern
that is used by the pen, pad, and touch interfaces. A "mapping" function
is provided to set up the battery, an "event" function is provided to
update the battery data, and a "report" function is provided to notify
the power_supply subsystem after all the data has been read. We look at
the usage itself rather than its collection to determine if one of the
battery functions should handle it. Additionally, we unconditionally
call the "report" function since there is no particularly good way to
know if a report contained a battery usage; 'wacom_notify_battery()'
will filter out any duplicate updates, however.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>