HID: thingm: fix workqueue race on remove

thingm_remove_rgb() needs to flush the workqueue after all the LED classes
have been unregistered, otherwise the removal might race with another LED
event coming, causing thingm_led_set() to schedule additional work after
thingm_remove_rgb() has flushed it. This obviously causes oops later, as
the scheduled work has been freed in the meantime.

In addition to that, move the hid_hw_stop() to an earlier place, so that
dmesg is not polluted by failure messages about not being able to write
the LED while the device is being shut down.

Reported-and-tested-by: Dylan Alex Simon <dylan-kernel@dylex.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>