UPSTREAM: Bluetooth: hci_ldisc: Fix null pointer derefence in case of early data

UPSTREAM: Bluetooth: hci_ldisc: Fix null pointer derefence in case of early data

HCI_UART_PROTO_SET flag is set before hci_uart_set_proto call. If we
receive data from tty layer during this procedure, proto pointer may
not be assigned yet, leading to null pointer dereference in rx method
hci_uart_tty_receive.

This patch fixes this issue by introducing HCI_UART_PROTO_READY flag in
order to avoid any proto operation before proto opening and assignment.

Signed-off-by: Loic Poulain <loic.poulain@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Change-Id: Ibe366f3222cbe7a093cd08aaecbc0de1004088c8
Signed-off-by: Jacob Chen <jacob2.chen@rock-chips.com>
(cherry picked from commit 84cb3df02aea4b00405521e67c4c67c2d525c364)