net: dsa: realtek: fix memory leak in rtl8366rb_setup_led()

net: dsa: realtek: fix memory leak in rtl8366rb_setup_led()

led_classdev_register_ext() only reads init_data.devicename - it never
stores the pointer. However, the caller allocated devicename with
kasprintf() but never freed it, leaking the string memory.

Fix it with a stack buffer to avoid dynamic buffers completely.

Fixes: 32d617005475 ("net: dsa: realtek: add LED drivers for rtl8366rb")
Signed-off-by: David Yang <mmyangfl@gmail.com>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260618140200.1888707-1-mmyangfl@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>