leds: leds-lm3530: Fix incorrect spelling of 'brightness'

Fixes the following W=1 kernel build warning(s):

 drivers/leds/leds-lm3530.c:113: warning: Function parameter or member 'brightness' not described in 'lm3530_data'

Cc: Pavel Machek <pavel@ucw.cz>
Cc: Kumar SAHU <shreshthakumar.sahu@stericsson.com>
Cc: Dan Murphy <D.Murphy@motorola.com>
Cc: linux-leds@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>