HID: wacom - add touch_arbitration parameter to wacom module

Touch arbitration is always on in wacom.ko. However, there are
touch enabled applications use both pen and touch simultaneously.
We should provide an option for userland to decide if they want
arbitration on or off.

This patch sets default touch_arbitration to on since most userland
apps are not ready to process pen and touch events simultaneously.
In the future, when userland is ready to accept pen and touch events
together, we will switch default touch_arbitration to off.

Tested-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>