Input: stmfts - acknowledge that setting brightness is a blocking call

We need to turn regulators on and off when switching brightness, and
that may block, therefore we have to set stmfts_brightness_set() as
LED's brightness_set_blocking() method.

Fixes: 78bcac7b ("Input: add support for the STMicroelectronics FingerTip touchscreen")
Acked-by: Andi Shyti <andi@etezian.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>