wifi: rtlwifi: use eth_broadcast_addr() to assign broadcast address

Use eth_broadcast_addr() to assign broadcast address instead
of memcpy().

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230804090120.1332091-1-yangyingliang@huawei.com