mt76: mt7615: Use after free in mt7615_mcu_set_bcn()

We dereference "skb" when we assign:

	req.pkt_len = cpu_to_le16(MT_TXD_SIZE + skb->len);
                                                ^^^^^^^^
So this patch just moves the dev_kfree_skb() down a bit to avoid the
use after free.

Fixes: 04b8e659 ("mt76: add mac80211 driver for MT7615 PCIe-based chipsets")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>