HID: thingm: set the proper error code before leaving

In case of an unsupported firmware, the driver bails out without setting
the LEDs interfaces, but forget to set the proper error code.
err is then still equal to 0 and the hid subsytem consider the device
to be in perfect shape.
When removing it, thingm_remove() tries to unbind the rgb LEDs which
has not been created, leading to a segfault.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>