drivers/video/backlight/s6e63m0.c: set permissions on gamma_table file to 0444

gamma_table is not writable, so set permissions to 0444.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
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>