The internal check_fb callback from struct pwm_bl_data is never
implemented. The driver's implementation of check_fb always
returns true, which is the backlight core's default if no
implementation has been set. So remove the code from the driver.

v2:
	* reword commit message
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20240305162425.23845-7-tzimmermann@suse.deSigned-off-by: Lee Jones <lee@kernel.org>