drivers/video/backlight/s6e63m0.c: fix corruption storing gamma mode

strict_strtoul() writes a long but ->gamma_mode only has space to store an
int, so on 64 bit systems we end up scribbling over ->gamma_table_count as
well.  I've changed it to use kstrtouint() instead.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>