udev-builtin-input_id: any i2c mouse is a pointing stick

Where we have a device that looks like a mouse and is connected over i2c, tag
it as pointing stick. There is no such thing as a i2c mouse.

Even touchpads that aren't recognized by the kernel will not show up as i2c
mouse - either the touchpad follows the Win8.1 specs in which case the kernel
switches it to multitouch mode and it shows up like a touchpad. The built-in
trackpoint, if any, is then the i2c mouse device.

Where the touchpad doesn't follow the spec, the kernel will not handle it and
the touchpad remains on the PS/2 legacy bus - not i2c. Hence we can assume
that any i2c mouse device is really a pointing stick.