rpmsg: virtio: Fix the unregistration of the device rpmsg_ctrl

rpmsg: virtio: Fix the unregistration of the device rpmsg_ctrl

Unregister the rpmsg_ctrl device instead of just freeing the
the virtio_rpmsg_channel structure.
This will properly unregister the device and call
virtio_rpmsg_release_device() that frees the structure.

Fixes: 6908263ba51c ("rpmsg: virtio: Register the rpmsg_char device")
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Reviewed-by: Hangyu Hua <hbh25y@gmail.com>
Link: https://lore.kernel.org/r/20220426060536.15594-4-hbh25y@gmail.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>