Bluetooth: hci_sync: Fix overwriting request callback

In a few cases the stack may generate commands as responses to events
which would happen to overwrite the sent_cmd, so this attempts to store
the request in req_skb so even if sent_cmd is replaced with a new
command the pending request will remain in stored in req_skb.

Fixes: 6a98e383 ("Bluetooth: Add helper for serialized HCI command execution")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>