Input: wacom_w8001 - handle errors from input_mt_init_slots()

input_mt_init_slots() may fail and we should be handling failures properly.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>