backlight: lp8788: staticize 'default_bl_config'

Fix the following sparse warning:

  drivers/video/backlight/lp8788_bl.c:55:25: warning: symbol 'default_bl_config' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>