Fix segfault with WMR controllers and SteamVR-Monado plugin

With the Reverb G2 controllers connected to the Linux host system via
Bluetooth the following segmentation fault occurs for me when starting
SteamVR with the SteamVR-Monado plugin installed:

```
$ gdb ...
...
(gdb) bt
#0 0x0000000000000000 in ?? ()
#1 0x00007f82de21bac7 in CDeviceDriver_Monado_Controller::RunFrame (this=0x7f82bc610e40)
at /home/linus/dev-priv/vr/monado/src/xrt/state_trackers/steamvr_drv/ovrd_driver.cpp:966
#2 0x00007f82de218b5d in CServerDriver_Monado::RunFrame (this=0x7f82de7c6ca0 <g_serverDriverMonado>)
at /home/linus/dev-priv/vr/monado/src/xrt/state_trackers/steamvr_drv/ovrd_driver.cpp:1574
#3 0x00000000004b7a77 in ?? ()
#4 0x0000000000000000 in ?? ()
(gdb) l /home/linus/dev-priv/vr/monado/src/xrt/state_trackers/steamvr_drv/ovrd_driver.cpp:966
959 if (m_xdev->hand_tracking_supported && m_skeletal_input_control.control_handle) {
960 vr::VRBoneTransform_t bone_transforms[OPENVR_BONE_COUNT];
961
962 timepoint_ns now_ns = os_monotonic_get_ns();
963 struct xrt_hand_joint_set out_joint_set_value;
964 uint64_t out_timestamp_ns;
965
966 m_xdev->get_hand_tracking(m_xdev,
967 m_hand == XRT_HAND_LEFT ? XRT_INPUT_GENERIC_HAND_TRACKING_LEFT
968 : XRT_INPUT_GENERIC_HAND_TRACKING_RIGHT,
969 now_ns, &out_joint_set_value, &out_timestamp_ns);
```

This happens because the "m_xdev->hand_tracking_supported" flag is set
but m_xdev->get_hand_tracking() is not implemented for WMR controllers.

Fixing this crash by setting hand_tracking_supported to false for
WMR controllers for now until get_hand_tracking() is implemented.

Link: https://gitlab.freedesktop.org/monado/monado/-/issues/251
Fixes: c4db3dfccc4a ("d/wmr: Add basic Reverb (G1, Bluetooth) motion controller support.")
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>