backlight: Constify struct backlight_ops

drivers/video/backlight/adp5520_bl.c

drivers/video/backlight/adx_bl.c

drivers/video/backlight/atmel-pwm-bl.c

drivers/video/backlight/backlight.c

drivers/video/backlight/corgi_lcd.c

drivers/video/backlight/cr_bllcd.c

drivers/video/backlight/da903x_bl.c

drivers/video/backlight/generic_bl.c

drivers/video/backlight/hp680_bl.c

drivers/video/backlight/jornada720_bl.c

drivers/video/backlight/kb3886_bl.c

drivers/video/backlight/locomolcd.c

drivers/video/backlight/mbp_nvidia_bl.c

drivers/video/backlight/omap1_bl.c

drivers/video/backlight/progear_bl.c

drivers/video/backlight/pwm_bl.c

drivers/video/backlight/tosa_bl.c

drivers/video/backlight/wm831x_bl.c

include/linux/backlight.h