Bluetooth: Classic: L2CAP: Handle multi L2CAP packets of a HCI ACL

In current implementation, if the HCI ACL data length exceeds on L2CAP
packet, the HCI ACL data will be discarded.

Support the case if the transport is classic.

Add a function `bt_br_acl_recv()` to handle the multi L2CAP packets
one by one.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>