HID: wacom: set stylus_in_proximity before checking touch_down

In wacom_bpt_pen, we checked touch_down before assigning new
stylus_in_proximity value. This would cause stylus_in_proximity not updated
properly if touch is down before pen is in proximity.

[jkosina@suse.cz: fix if-else style]
Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>