Input: usbtouchscreen - extend coordinate range for Generaltouch devices

Generaltouch protocol allows for coordinates in [0, 0xffff] range and
there are devices reporting coordinates as high as 0x7fff so let's update
the driver to reflect that.

Signed-off-by: Roy Yin <yhch@generaltouch.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>