Plane: Tiltrotor: fix has_vtol_motor function

[IamPete1]

This fixes a by reported by @jcproulx in #29404.

It was always checking the first motor rather than any motor. The _have_vtol_motor bool is used in the has_vtol_motor function which is only used here:

ardupilot/ArduPlane/quadplane.cpp

Lines 1591 to 1594 in 076951f

	if (quadplane.tiltrotor.enabled() && !quadplane.tiltrotor.has_vtol_motor() && !quadplane.tiltrotor.has_fw_motor()) {
	// All motors tilting, Use a combination of vertical and forward throttle based on curent tilt angle
	// scale from all VTOL throttle at airspeed_reached_tilt to all forward throttle at fully forward tilt
	// this removes a step change in throttle once assistance is stoped

The goal of that code is to smooth the handover of throttle control from VTOL to FW in the transition timer phase on vehicles that had all motors tilting.