backlight: lp855x: Catch errors when changing brightness

backlight: lp855x: Catch errors when changing brightness

The lp855x_bl_update_status function's return type is int, but
it always returns 0, without checking for the results of the
write_byte/pwm_ctrl functions called within.

Make this function return the return values of the functions it
calls, and modify the lp855x_pwm_ctrl function to return errors.

Signed-off-by: Artur Weber <aweber.kernel@gmail.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20230714121440.7717-3-aweber.kernel@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>