Bluetooth: Userchan: Parse multiple hci packets

After conducting tests with a a virtual Bluetooth controller
over TCP it was noticed that some HCI packets may arrive on the
same buffer if sent over a short period of time.
This update ensures the hci packets are parsed correctly in the case
multiple packets are in the same recieving buffer according to
the Bluetooth Spec v5.4 Part E.

Signed-off-by: Victor Chavez <chavez-bermudez@fh-aachen.de>