HID: wacom: fix proximity tool release

Don't zero the current tool before reporting its release to the input
subsystem.

Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Tested-by: Przemo Firszt <przemo@firszt.eu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>