Bluetooth: hci_serdev: clear HCI_UART_PROTO_READY to avoid closing proto races

Clearing HCI_UART_PROTO_READY will avoid usage of proto function pointers
before running the proto close function pointer. There is chance of kernel
crash, due to usage of non proto close function pointers after proto close.

Signed-off-by: Balakrishna Godavarthi <bgodavar@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>