leds: mlxreg: Fix possible buffer overflow

Error was detected by PVS-Studio:
V512 A call of the 'sprintf' function will lead to overflow of
the buffer 'led_data->led_cdev_name'.

Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Oleh Kravchenko <oleg@kaa.org.ua>
Signed-off-by: Pavel Machek <pavel@ucw.cz>