Bluetooth: Use hci_recv_stream_fragment() in UART driver

Use the new hci_recv_stream_fragment() to reassembly incoming UART
streams.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Tested-by: Ville Tervo <ville.tervo@nokia.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>