HID: wacom: Use calculated pkglen for wireless touch interface

Commit 01c846f9 introduced the 'wacom_compute_pktlen' function which
automatically determines the correct value for an interface's pkglen
by scanning the HID descriptor. This function returns the correct
value for the wireless receiver's touch interface, removing the need
for us to set it manually here.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>