Bluetooth: btusb: Improve stability for QCA devices

WCN6855 2.1 will reset to apply firmware downloaded, so wait
a moment for reset done then go ahead to improve stability.

Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>