Input: wacom - use in-kernel HID parser

HID already parses the report descriptor, so use it instead of implementing
our own.  The special case for Bamboo PT 3rd gen is also removed and
handled in the same way Intuos 5 is treated, by hardcoding it in the
driver.  Last, the unit_exponent stored into the hid field already is
signed, so there is no need to handle a two's complement anymore.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Tested-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>