Bluetooth: hci_qca: Disable IBS state machine and flush Tx buffer

During hci down we observed IBS sleep commands are queued in the Tx
buffer and hci_uart_write_work is sending data to the chip which is
not required as the chip is powered off. This patch will disable IBS
and flush the Tx buffer before we turn off the chip.

Signed-off-by: Balakrishna Godavarthi <bgodavar@codeaurora.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>