Bluetooth: hci_qca: Skip serdev wait when no transfer is pending

qca_suspend() calls serdev_device_wait_until_sent() regardless of
whether a transfer is pending. While it does no active harm since
the function should return immediately it makes the code more
confusing. Add a flag to track whether a transfer is pending and
only call serdev_device_wait_until_sent() is needed.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>