Bluetooth: btusb: use skb_pull to avoid unsafe access in QCA dump handling

Use skb_pull() and skb_pull_data() to safely parse QCA dump packets.

This avoids direct pointer math on skb->data, which could lead to
invalid access if the packet is shorter than expected.

Fixes: 20981ce2 ("Bluetooth: btusb: Add WCN6855 devcoredump support")
Signed-off-by: En-Wei Wu <en-wei.wu@canonical.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>