drivers/video/backlight/kb3886_bl.c: use usleep_range() instead of msleep() for small sleeps

Since msleep() might not sleep for the desired amount when less than 20ms,
use usleep_range().

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Claudio Nieder <private@claudio.ch>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>