Bluetooth: btusb: fix bt fiwmare downloading failure issue for qca btsoc.

This is btsoc timing issue, after host start to downloading bt firmware,
ep2 need time to switch from function acl to function dfu, so host add
20ms delay as workaround.

Signed-off-by: Tim Jiang <tjiang@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>