Input: synaptics-rmi4 - when registering sensors do not call them "drivers"

We are not registering drivers, but transport devices (AKA sensors), so
let's call them that.

Also let's rename "retval" to "error" in probe() functions as the variables
are used to store error codes.

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