video/backlight: s6e63m0: Fix string type mismatch

Fix string type mismatch in s6e63m0_sysfs_show_gamma_table().
gamma_table_count is defined as unsigned int.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>