Bluetooth: hci_qca: wait for Pre shutdown complete event before sending the Power off pulse

When SoC receives pre shut down command, it share the same
with other COEX shared clients. So SoC needs a short time
after sending VS pre shutdown command before turning off
the regulators and sending the power off pulse. Along with
short delay, needs to wait for command complete event for
Pre shutdown VS command

Signed-off-by: Harish Bandi <c-hbandi@codeaurora.org>
Reviewed-by: Balakrishna Godavarthi <bgodavar@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>