Input: wacom - override 'pressure_max' with value from HID_USAGE_PRESSURE

The 0xEC sensor is used in multiple tablet PCs and curiously has versions
that report 256 levels of pressure (Samsung Slate 7) as well as versions
that report 1024 levels (Lenovo Thinkpad Yoga).  To allow both versions to
work properly, we allow the value of HID_USAGE_PRESSURE reported to
override pressure_max.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Tested-by: Aaron Skomra <Aaron.Skomra@wacom.com>
Reviewed-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>