HID: wacom: Force pen out of prox if no events have been received in a while

Prox-out events may not be reliably sent by some AES firmware. This can
cause problems for users, particularly due to arbitration logic disabling
touch input while the pen is in prox.

This commit adds a timer which is reset every time a new prox event is
received. When the timer expires we check to see if the pen is still in
prox and force it out if necessary. This is patterend off of the same
solution used by 'hid-letsketch' driver which has a similar problem.

Link: https://github.com/linuxwacom/input-wacom/issues/310
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>