HID: wacom: generic: sync pad events only for actual packets

Commits d793ff81 and 4082da80 introduced two pad usages which do not
actually send pad input events. To make sure we do not post empty
pad packets, pad_input_event_flag is introduced. Turn on the flag
for real pad input events so we can synchronize them properly.

Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>