HID: wacom: make sure touch arbitration is applied consistently

stylus_in_proximity is used to make sure no touch event is sent while pen is in
proximity. touch_down is used to make sure a touch up event is sent when pen
comes into proximity while touch is down.

Two touch routines didn't store touch_down. One touch routine forgot to check
stylus_in_proximity before sending touch events. This patch fixes those issues.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>