Input: wacom - ignore unwanted bamboo packets

Bamboo's Pen and Touch packets always start with a value
of 0x02 in first byte.  In 3rd gen Bamboo's, the hw is now
periodically sending some additional packets with unrelated data
and uses a value other than 0x02 to inform driver this.

Ignore those packets now.

This was reported by users as bad behavior in Gimp. The
invalid packets being processed made the stylus report
out of proximity for the 1 packet and this triggered some
secondary bug which caused Gimp to stop drawing until
user really took pen out of proximity of tablet.

Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>