HID: thingm: remove not needed error message

LED core takes care of handling failed calls to thingm_let_set.
- print error message in set_brightness_delayed or
- pass error to caller in led_set_brightness_sync
Also the error message here doesn't provide any hint what actually
went wrong. Therefore remove it.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@rehat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>