Input: wacom - don't retrieve touch_max when it is predefined

Some models, such as 0xE6, report more fingers than we process.

Reported-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Tested-by: Nils Kanning <nils@kanning.de>
Tested-by: Rafi Rubin <rafi@seas.upenn.edu>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>