HID: wacom: do not directly use input_mt_report_pointer_emulation

input_mt_sync_frame() calls input_mt_report_pointer_emulation() and do
some internal steps required to keep in sync the state of the touch within
the various reports.

Given that we use input_mt_get_slot_by_key() in this driver, it is better to
use input_mt_sync_frame().

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