HID: wacom: insert timestamp to packed Bluetooth (BT) events

To fully utilize the BT polling/refresh rate, a few input events
are sent together to reduce event delay. This causes issue to the
timestamp generated by input_sync since all the events in the same
packet would pretty much have the same timestamp. This patch inserts
time interval to the events by averaging the total time used for
sending the packet.

This decision was mainly based on observing the actual time interval
between each BT polling. The interval doesn't seem to be constant,
due to the network and system environment. So, using solutions other
than averaging doesn't end up with valid timestamps.

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