Input: wacom_w8001 - ignore invalid pen data packets

ThinkPad X60 Tablet PC (pen only device) sometime posts
packets that are larger than W8001_PKTLEN_TPCPEN.

Reported-by: Chris J Arges <christopherarges@gmail.com>
Tested-by: Chris J Arges <christopherarges@gmail.com>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>