Input: synaptics-rmi4 - log when we create a guest serio port

To ease analyzing boot behavior from logs, let's log when we are about to
register the pass-through serio port.

Also, let's drop "Synaptics" prefix from the port name, as RMI4 is good
enough indicator already, and having the prefix means that the name does
not fit into serio->name field. While at it move from hard-coded seio->phys
to one mentioning the sensor ID (such as "rmi4-00.fn03/serio0").

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>