drivers/video/backlight/l4f00242t03.c: make 1-bit signed field unsigned

Fixes sparse warning:
drivers/video/backlight/l4f00242t03.c:28:21: error: dubious one-bit signed bitfield

Signed-off-by: Mariusz Kozlowski <mk@lab.zgora.pl>
Acked-by: Richard Purdie <rpurdie@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>