Input: tm2-touchkey - acknowledge that setting brightness is a blocking call

We need to access I2C bus when switching brightness, and that may block,
therefore we have to set stmfts_brightness_set() as LED's
brightness_set_blocking() method.

Fixes: 72d1f234 ("Input: tm2-touchkey - add touchkey driver support for TM2")
Acked-by: Andi Shyti <andi@etezian.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>